REINFORCED  CONCRETE  BUILDINGS 


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REINFORCED 
CONCRETE   BUILDINGS 


A  TREATISE  ON  THE  HISTORY,  PATENTS 
DESIGN  AND  ERECTION  OF  THE  PRINCI- 
PAL PARTS  ENTERING  INTO  A  MODERN 
REINFORCED  CONCRETE  BUILDING 


BY 
ERNEST   L.    RANSOME 

Assoc.  Am.  Soc.  C.  E.,  Charter  Member,  W.  Soc.  E.,  Hon.  Corres. 

Member,  A.  I.  A.,  Member,  Royal  Society   of  Arts,   President   and 

Consulting  Engineer,  The  Ransome  Engineering  Company 

AND 

ALEXIS  SAURBREY 

Assoc.  M.  Am.  Soc.  C.  E.,  Member,  Dansk  Ingenior  forening,  Mana- 
ger and  Chief  Engineer,  The  Ransome  Engineering  Company 


McGRAW-HILL  BOOK  COMPANY 

239  WEST  39TH  STREET,  NEW  YORK 
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1912 


Copyright,  1912y  by  McGRAW-HiLL  BOOK  COMPANY 

6 


THE  -PLIMPTON  -PRESS  -NORWOOD  -MASS  -U-S -A 


PREFACE 

THIS  little  volume  is  presented  to  the  engineering  profession 
for  the  purpose  of  showing  what  reinforced  concrete  is,  and  how 
it  came  to  be  what  it  is.  In  deciding  upon  the  scope  of  the  book, 
the  authors  have  endeavored  to  select  matters  of  interest  to  the 
mature  and  experienced  engineer,  and  for  that  reason  the  ultimate 
result  of  the  analysis  has  been  treated  in  greater  detail  than  the 
derivation  itself. 

Many  books  on  reinforced  concrete  have  been  written  prin- 
cipally for  the  practical  man  or  even  for  the  untrained  man;  those 
who  approve  of  that  tendency  will  not  approve  of  this  book,  in 
which  all  references  to  weights  and  dimensions,  earth  pressure, 
etc.,  have  been  avoided.  The  practicing  engineer,  the  contractor, 
or  even  the  college  student  should  look  for  such  matters  in  special 
"pocket  books,"  and  he  must  not  expect  the  reinforced  concrete 
book  to  furnish  a  complete  encyclopedia  on  civil  and  hydraulic 
engineering. 

On  the  other  hand,  much  matter  has  been  included  in  this 
book  which  will  be  looked  for  in  vain  in  other  works,  and  this 
is  especially  true  of  Part  I,  where  an  account  of  the  history  of 
reinforced  concrete  has  been  given,  with  special  reference  to  the 
patents  granted  by  the  United  States.  Naturally,  a  selection  of 
the  more  important  or  interesting  patents  is  a  difficult  matter, 
and  most  likely  some  readers  will  find  that  too  much  has  been 
included,  and  others  that  not  enough  has  been  included.  How- 
ever this  may  be,  the  records  of  the  patent  office  contain 
on  the  whole  more  and  better  information  than  any  other 
source,  and  the  engineer  who  is  striving  to  attain  perfection 
cannot  afford  to  relegate  the  most  valuable  thoughts  and  results 
of  his  predecessors  to  the  scrap  heap.  It  is  also  hoped  that 
inventors  and  patent  attorneys  may  find  matters  of  interest  in 
the  necessarily  brief  descriptions  given;  anybody  wishing  full 
information  in  regard  to  any  patents  may  obtain  copies  for 

vii 


241330 


viii  PREFACE 

a  nominal  sum  by  addressing  the  Commissioner  of  Patents, 
Washington, .  B.C. 

The  theoretical  analysis  in  Part  II  has  been  made  as  brief  and 
concise  as  seemed  consistent  with  its  purpose;  —  the  solutions  of 
equations,  etc.,  have  usually  been  given  after  the  premises  have 
been  stated,  omitting  all  the  intermediate  steps.  If  this  book 
should  find  its  way  into  the  classroom,  the  teacher  can  easily 
supply  what  is  omitted;  —  the  practical  engineer  would  not  stop 
to  read  a  treatise  on  mathematics,  in  any  case.  The  author  be- 
lieves that  much  of  this  is  new  and  original,  especially  the  use  of 
two  constants  in  the  bending  problem;  the  further  development 
in  Articles  34,  35,  and  49,  where  the  effect  of  an  increase  in  depth 
of  beam  is  discussed,  and  the  analysis  of  stresses  in  given  beams. 

The  entire  chapter  on  Transverse  Stresses  and  U-bars  is  origi- 
nal, and  avoids  the  use  (or  mis-use)  of  the  word  "  shear."  In 
reinforced  concrete  the  steel  is  supposed  to  act  in  tension,  and  the 
U-bars  must  follow  this  general  rule. 

Part  III  is  devoted  to  the  practical  construction.  Here  again 
more  attention  has  been  given  to  the  useful  facts  not  generally 
known,  than  to  those  that  are  matters  of  common  knowledge. 
There  is  no  necessity  today  for  describing  at  great  length  the 
various  types  of  buildings,  or  their  component  parts;  an  excep- 
tion has  however  been  made  in  regard  to  "Unit  Construction" 
which  appears  to  be  coming  rapidly  to  the  front.  No  effort  has 
been  made  toward  giving  the  details  of  form  design,  but  the 
general  principles  have  been  stated  with  great  care.  The  chap- 
ters on  fireproofing  and  repairs  should  be  of  interest,  and  the 
superintendents'  specifications  have  proved  their  own  value  on 
a  number  of  large  contracts.  Immediately  preceding  this  chap- 
ter we  have  placed  a  short  account  of  some  bad  failures;  while 
we  have  not  been  able  to  throw  new  light  on  the  causes,  we  hope 
that  the  perusal  of  the  chapter  on  "accidents"  may  put  the  reader 
in  the  proper  frame  of  mind  to  not  only  read,  but  also  follow,  the 
instructions  given  in  the  last  chapter. 

It  has  been  customary  with  other  writers  to  describe  in  more 
or  less  detail  the  tests  made  on  reinforced  concrete  beams.  As 
a  general  principle  we  have  avoided  such  discussions,  partly 
because  the  plan  of  this  book  did  not  allow  us  to  devote  the 
required  large  number  of  pages,  and  partly  because  the  vast 
majority  of  tests  are  of  little  value,  not  from  want  of  ability  or 


PREFACE  ix 

care  in  the  experimenters,  but  because  the  tests  were  not  sys- 
tematized, that  is,  every  group  should  first  demonstrate  one 
general  fact,  and  then  individual  test  specimens  should  be  so 
designed  that  they  vary  in  only  one  feature  from  the  standard, 
so  that  the  effect  of  the  variation  at  once  becomes  evident.  The 
groups  of  tests  so  made  are  few  indeed,  and  only  during  the  last 
few  years  have  clear  photographs  of  the  broken  specimens  been 
published.  However,  where  a  given  problem  requires  the  illus- 
tration of  a  test,  the  best  available  source  has  been  referred  to. 

In  this  book  an  earnest  effort  has  been  made  toward  stating 
the  truth  when  it  was  known,  and  to  make  it  clear  and  evident 
that  the  truth  is  not  known  in  a  number  of  cases.  The  chapters 
relating  to  the  mathematical  design  are  so  arranged  that  every- 
one can  readily  assure  himself  of  the  correctness,  but  in  regard  to 
such  matters  as  cement  testing,  rolling  of  concrete  floors  while 
setting,  and  numerous  other  practical  or  general  propositions 
where  the  authors  have  taken  issue  with  prevailing  ideas,  and 
gone  contrary  to  accepted  practice,  our  statements  must  either 
be  rejected  as  heresy  or  accepted  as  doctrine. 

The  authors  desire  to  acknowledge  their  indebtedness  to  vari- 
ous papers  published  in  the  Engineering  Record,  the  Engineering 
News,  the  American  Machinist,  and  the  Cement  Age.  Informa- 
tion has  also  been  gained  from  a  paper  read  by  Geo.  W.  Percy 
before  the  San  Francisco  Chapter  of  the  A.I.A.  (Feb.  9,  1894); 
from  C.  W.  Pasley's  "Observations  on  Limes"  (1847),  from 
Hyatt's  " Account  of  some  Experiments"  (1877);  from  papers  by 
Scott,  Bernays,  and  Grant,  edited  by  James  Forest  as  a  separate 
volume  under  the  name  " Portland  Cement"  (1880);  from  " Re- 
inforced Concrete  in  Factory  Construction"  by  the  Atlas  Portland 
Cement  Co.,  and  in  regard  to  theoretical  questions,  from  works 
by  Considere  and  Morsch.  The  author  of  Part  II  desires  to  em- 
phasize the  inspiration  received  from  the  study*  of  these  two 
authorities,  who  have  contributed  so  much  to  the  knowledge  of 
the  subject.  The  discovery  of  the  "  water-marks "  by  Professor 
Turneaure  has  perhaps  influenced  the  U-bar  theory  here  advanced 
more  than  any  other  tests  on  record  have. 

The  authors  are  greatly  indebted  to  Professor  L.  J.  Johnson, 
M.  Am.  Soc.  C.  E.,  for  certain  data  relating  to  reinforced  concrete 
beams  tested  at  Harvard  University.  The  results  obtained 
are  extremely  important  and  will  undoubtedly  revolutionize 


x  PREFACE 

current  practice  in  regard  to  the  simultaneous  manufacture  of 
beam  and  slab.  Moreover,  these  tests  confirm  in  a  remark- 
able manner  the  theories  advanced  in  Chapter  VII,  which 
were  conceived  and  printed  long  before  the  test  beams  were 
designed. 

E.  L.  R. 

A.  S. 
MARCH,  1912. 


CONTENTS 


PART   I 

CONTRIBUTION  TO   THE   HISTORY  OF  REINFORCED 
CONCRETE   CONSTRUCTION 

CHAPTER  PAGE 

I     PERSONAL  REMINISCENCE  BY  ERNEST  L.  RANSOME       ...         1 

The  invention  of  "Ransome  Stone"  in  England,  1844. — 
Introduction  in  America  in  1870.  —  Early  work  in  Portland 
Cement  Concrete  on  the  Pacific  Coast.  —  The  invention  of 
twisted  bars  (1884).  —  Reinforced  Concrete  Buildings  in  and 
near  San  Francisco.  —  Their  behavior  in  the  Earthquake.  — 
The  first  "ribbed"  floors  (1889).  —  Effect  of  retempering  of 
mortar  and  of  continued  mixing.  —  Rolling  the  floors.  — 
Development  of  Sidewalk  Lights.  —  Joining  new  and  old  con- 
crete. —  Concrete  made  fireproof  by  adding  salt,  waterproof 
by  adding  lime;  effect  of  clay  in  the  aggregates.  —  The  Re- 
inforced Concrete  Belt  Course,  its  use  and  advantages.  —  The 
Coil  Joint  for  joining  Lapping  Bars.  —  Notes  on  falsework,  the 
importance  of  standardization;  floor  core  boxes;  unit  con- 
struction.—  "Wet"  and  "Dry"  concrete.  —  Injurious  Agen- 
cies. —  The  growth  of  Reinforced  Concrete  Construction. 

II    BASIC  PATENTS,  AND  A  SHORT  SURVEY  OF  THE  EARLY  HISTORY 

OF  THE  ART  BY  ALEXIS  SAURBREY 18 

Elements  of  invention.  —  Ancient  use.  —  Early  ideas  of  the 
properties  of  lime.  —  Smeaton's  chemical  analysis.  —  Discovery 
of  "Roman  Cement"  and  "Portland  Cement." 

THE  PERIOD  OF  DISCOVERY 19 

Cement  testing  a  necessity.  —  Brunei's  Reinforced  Brick 
Arches  (1834-1838)  and  Beams.  —  Pasley's  Beams.  —  Ranger's 
patent  and  early  works.  —  Early  English  patents.  —  Edward's 
Analysis  and  Patents.  —  Use  of  Reinforced  Concrete  in  England, 
in  Germany,  in  Holland,  in  France.  —  Early  American  patents. 
-  Hyatt's  patent  of  1878. 

THE  PERIOD  OF  IMPROVEMENT 35 

RECENT  PATENTS 40 

xi 


xii  CONTENTS 

PART   II 

RATIONAL   DESIGN   OF  REINFORCED   CONCRETE   BUILDINGS 
BY  ALEXIS  SAURBREY 

CHAPTER  PAGE 

III  INTRODUCTION 51 

Definition;  homogeneity;  mutual  relation  of  steel  and  con- 
crete; assumptions;  principles;  the  necessity  of  "  bond." 

IV  ADHESION 54 

Laws;  anchorage  in  beams;  diameter  of  rod. 

V    COMPRESSION  AND  LATERAL  EXPANSION 57 

Laws;  effect  and  necessity  of  hooping;  calculation  of  plain 
and  hooped  columns;  least  diameter;  practical  considerations. 

VI    BENDING 66 

Notations.  —  Assumptions.  —  Design.  —  Slab  Formulas.  — 
T-beams.  —  Tile-Concrete-Construction.  —  Simplified  Formulas 
for  flat  slabs.  —  Discussion  of  the  tables;  numerical  examples. 
—  Analysis  of  given  beams;  numerical  examples. 

VII    TRANSVERSE  STRESSES 92 

Is  a  concrete  beam  solid? — -Effect  of  hair  cracks.  —  Effect 
of  large  cracks.  —  Adjustment  of  stresses  to  meet  the  new  con- 
dition in  a  cracked  beam.  —  The  U-bar  as  cantilever  reinforce- 
ment. —  Calculation.  —  Same  result  obtained  in  a  simpler  but 
less  convincing  manner.  —  The  equilibrium  curve  must  be 
approximated.  —  Moving  and  stationary  loads.  —  How  many 
bars  should  be  bent  up?  —  Anchorage  a  necessity  for  rational 
calculation.  —  Spacing  of  U-bars.  —  Shear  must  be  considered 
as  a  component  stress  resulting  from  tension  and  compression 
acting  simultaneously.  —  Frictional  stresses.  —  Beams  and 
slabs  not  monolithic.  —  Angle  of  friction.  —  That  the  same  rule 
should  be  used  whether  beams  are  ca&  in  one  piece  with  the 
slab  or  not.  —  Tensile  stresses  disregarded.  —  The  true  assump- 
tion. —  Details  of  reinforcement;  the  elements  combined  to  one 
beam. 

VIII    APPLICATIONS  OF  THE  BENDING  THEORY 109 

Continuity.  —  Moment  of  Inertia;  double  reinforcement; 
Combined  Bending  and  Compression ;  a  simple  chimney  formula  ; 
footings ;  circular  reinforcement  in  plates ;  theory  of  plates. 

IX    INITIAL  AND  ALLOWABLE  STRESSES 126 

Setting  in  air;  in  water;  wetting  dry  concrete;  shrinkage; 
temperature  stresses;  expansion  joints.  —  Assumptions;  factor 
of  safety;  allowable  stresses  fixed  by  custom.  —  Columns,  floors; 
other  structures;  actual  safety. 


CONTENTS  xiii 

PART   III 

PRACTICAL  CONSTRUCTION   BY  ERNEST  L.   RANSOME 
AND  ALEXIS  SAURBREY 

CHAPTER  PAGE 

X    MATERIALS  OF  CONSTRUCTION 137 

Cement :  tests  not  reliable.  —  Storage  and  use.  —  Quick  set- 
ting cement. —  Samples;  field  tests  of  concrete.  —  Sand:  tests; 
standard  sand;  specifications. — Stone:  dust;  run  of  crusher; 
separate  piles;  sizes  used;  furnace  slag;  boiler  cinders,  lime 
stone;  sandstone;  brick;  shale;  conglomerate.  —  Steel :  plain 
bars;  rerolled  bars;  deformed  bars;  wire  mesh.  —  Tiles.  — 
Concrete:  Mixing;  water  used;  deposited  informs;  supervision; 
joints;  cold  and  heat. 

XI    FLOOR  SYSTEMS 152 

Monolithic  vs.  Unit.  —  Design. — Monolithic  Work.  —  Forms; 
reinforcement. —  Unit  Work.  —  Where  used;  the  Ransome  Sys- 
tem. 

'  XII    FOUNDATIONS 171 

Brief  description;  piling. 

XIII  FINISHING  OPERATIONS 176 

Corners;  flat  surfaces;  plastering;  brick  and  terra  cotta; 
improved  surfaces;  tooling;  rubbing;  brushing;  left  unfinished. 
—  Floor  finish. 

XIV  FIREPROOFING  AND  FIRES 183 

Smoke  and  water.  —  Insurance.  —  Behavior  of  concrete;  thick- 
ness required;  protection  of  corners.  —  Salt  retards  attack  of 
fire.  —  Bayonne  fire.  —  Pittsburg,  Baltimore,  San  Francisco.  — 
Comparison  between  different  methods. 

XV    REPAIRS  TO  EXISTING  BUILDINGS 188 

Cracks  in  floors;  in  beams;  repairs  of  columns;  cutting  con- 
crete; laitance;  settling  of  footings;  hardening  soft  concrete. 

XVI    ACCIDENTS 191 

South  Framingham;   Long  Beach;   Rochester;   Philadelphia; 
Annapolis;    Saybrook;    Cleveland.  —  Defective  column  design. 
Three  good  rules. 

XVII    SUPERINTENDENTS  SPECIFICATIONS 195 

General;  temporary  offices  and  buildings,  setting  up  plant, 
etc.;  excavating  and  grading;  molds;  concrete;  steel;  finish- 
ing; acid  joint. 


xiv  CONTENTS 

XVIII  THE  ENGINEER 200 

Reinforced  Concrete  manufactured  in  situ.  —  Necessity  for 
skillful  manipulation.  —  Building  Ordinances;  in  Cleveland;  in 
Boston.  —  Uniform  Regulations.  —  Injurious  influences.  — "  Cost 
plus  profit"  Contract.  —  Lump  Sum  Contract.  —  Monthly  esti- 
mates.—  Qualifications  of  the  Engineer.  —  Patents. 

XIX    THE  THEORY  OF  BEAMS  AS  ILLUSTRATED  BY  TESTS    .      .      .     207 

Extensibility. —  Shear  Resistance. —  Stirrups. —  German  Tests 
on  T-Beams. — Author's  Tests  at  Case  School.  —  Professor 
Johnson's  Tests  at  Harvard. 


INDEX 233 


PART   I 

A  CONTRIBUTION  TO  THE  HISTORY  OF 
REINFORCED  CONCRETE 


REINFORCED   CONCRETE   BUILDINGS 

CHAPTER   I 

PERSONAL  REMINISCENCE 

BY  ERNEST  L.  RANSOME 

WHEN,  in  1859,  I  entered  as  an  apprentice  in  my  father's 
factory  in  Ipswich,  England,  the  concrete  industry  was  in  its 
infancy,  and  was  confined  largely  to  the  manufacture  of  arti- 
ficial stone  for  ornamental  purpose.  One  of  the  earliest  appli- 
cations of  the  new  industry  was  invented  in  1844  by  my  father 
Frederick  Ransome,  who  was  then  engaged  as  superintendent 
of  the  well-known  Iron  Works  of  Ransomes  and  Sims  at  Ipswich. 
Noticing  one  day  the  waste  of  good  hard  stone  in  the  dressing 
of  mill-stones,  he  conceived  the  idea  of  cementing  hard,  selected 
pieces  together,  and  so  to  manufacture  a  superior  grade  of  burr- 
stones.  The  first  difficulty  was  in  finding  a  proper  cementing 
substance:  plaster  of  paris,  shellac,  glue,  isinglass,  lime  with 
bullock's  blood,  mastic,  etc.,  were  tried  and  discarded.  Among 
the  numberless  ingredients  tried  were  also  common  glass,  but 
it  was  not  until  experiments  with  soluble  glass  were  made  that 
success  became  probable.  It  occurred  to  him  that  if  he  took 
flint  stones  with  a  moderate  amount  of  caustic  alkali  in  solution, 
and  subjected  them  to  heat  in  a  Papin's  digester  under  high 
pressure,  he  might  be  able  to  concoct  a  soup  from  flint,  as  Papin 
had  done  from  bones.  But  the  result  was  apparently  a  dis- 
appointment, and  in  order  to  increase  the  heat,  he  finally  tied 
the  safety  valve  with  a  piece  of  wire,  and  forced  the  fire  until 
the  boiler  became  overheated.  Fearing,  however,  that  the  boiler 
would  blow  up,  he  threw  it  out  into  a  cistern  with  cold  water, 
and  the  boiler,  as  might  have  been  anticipated,  was  broken  to 
pieces  —  and  there,  inside,  was  the  glazy,  syrupy  mass  of  dis- 
solved glass.  The  portions  next  to  the  walls  of  the  boiler  were 
baked  to  a  flinty  hard  stone;  in  one  word,  the  problem  was 
solved. 

1 


2  REINFORCED  CONCRETE  BUILDINGS 

Step  by  step,  a  process  was  now  evolved  whereby  a  cement- 
ing substance  was  had,  as  above  described;  and  based  upon  this 
process,  a  large  business  was  developed.  Before  long,  the 
parent  Company,  "  Patent  Concrete  Stone  Co.,"  was  selling 
its  product  in  all  parts  of  the  world,  especially  after  methods 
had  been  invented  whereby  the  stones  were  made  not  only  hard 
but  also  weather-proof.  This  process  consisted  originally  in  the 
application  of  a  solution  of  chloride  of  calcium  to  the  silicate  of 
soda  previously  used,  whereby  insoluble  silicate  of  lime,  and 
soluble  chloride  of  sodium  were  formed  by  double  decomposition. 
The  latter  is  common  cooking  salt  and  was  easily  removed  by 
washing. 

A  further  experiment  disclosed  the  fact  that  powdered  mag- 
nesian  limestone,  mixed  with  a  small  quantity  of  silicate  of 
soda,  formed  a  very  hard  substance  when  submerged  in  a  solu- 
tion of  chloride  of  calcium,  in  a  very  short  time. 

In  America,  the  new  process  was  introduced  in  1870  by  the 
Pacific  Stone  Company  of  San  Francisco,  of  which  Company  I 
was  the  superintendent  for  four  years.  About  this  time,  the 
concrete  industry  was  in  slow  development  on  the  Coast,  based 
upon  the  use  of  imported  Portland  Cement;  in  1874  I  remember 
to  have  paid  as  much  as  nine  dollars  per  barrel  of  cement.  But 
even  as  late  as  1882,  the  concrete  construction  was  mainly 
utilized  in  foundations  and  arches  suspended  between  iron  beams. 
In  the  latter  type  of  construction  some  trouble  was  experienced 
with  the  cracking  of  the  concrete  over  the  beams,  and  to  over- 
come this  tendency  I  patented,  No.  263,579  (Figure  1),  a  con- 


FIGURE  1. 

struction  in  which  an  expansion-joint  feature  was  introduced, 
and  several  sidewalks  have  been  built  over  cellar  areas  in  this 
manner. 

Before  long,  I  was  called  upon  to  devise  a  cheaper  method 
of  self-supporting  sidewalks  for  the  Masonic  Hall  at  Stockton, 
Cal.,  and  this  I  accomplished  by  using,  instead  of  the  I  beams, 
a  2"  round  tie-bolt  to  carry  the  tension,  while  the  concrete 


PERSONAL  REMINISCENCE  3 

carried  the  compression.  The  rods  had  upset  ends  and  large 
cast-iron  washers  at  each  end,  and  I  soon  found  that  the  up- 
setting and  threading  of  the  ends,  the  nuts  and  washers,  etc., 
made  the  cost  of  the  finished  rod  exactly  twice  that  of  the  plain 
rod.  I  looked  around  for  means  whereby  a  continuous  tie  or 
bond  could  be  developed  along  the  length  of  the  rod,  and  even 
contemplated  cutting  a  spiral  groove  in  the  rod,  when  suddenly 
the  idea  of  twisting  a  square  or  rectangular  bar  entered  my  head. 
I  happened  to  have  a  rubber  band  in  my  pocket,  and  the  spiral 
thread  became  at  once  evident  when  the  rubber  band  was 
twisted  in  the  hand.  My  patent,  No.  305,226  (Figure  2),  was 
granted  in  1884,  and  the  mills  were  soon  turning  out 
twisted  bars  up  to  one  inch  square,  at  a  cost  of  about 
ten  dollars  per  ton  for  twisting.  Larger  bars  they 
positively  refused  to  tackle  under  the  plea  that  the 
common  lathes  used  for  the  purpose  did  not  have  the 
requisite  strength.  I  had,  however,  in  my  yard  an  old 
concrete  mixer  equipped  with  a  worm  and  wheel,  and 
by  modifying  this  arrangement  I  soon  succeeded  in 
twisting  2"  square  rods,  using  hand  power.  The  cost 
did  not  exceed  seventy-five  cents  per  ton,  and  from 
that  date  until  a  more  recent  period,  all  the  twisting 
was  done  in  my  own  yards. 

However,  the  introduction  of  the  twisted  iron  was 
no  easy  matter,  and  when  I  presented  my  new  inven- 
tion to  the  technical  society  in  California,  I  was  simply 
laughed  down,  the  concensus  of  opinion  being  that  I 
injured  the  iron.     One  gentleman  kindly  suggested  that 
if  I  did  not  twist  my  iron  so  much  I  might  not  injure  it  seri- 
ously, in  spite  of  all  my  references  to  the  twisting  of  ropes  and 
similar   devices.     This   argument   I   based  upon  the  supposed 
fibrous  or  laminated  structure  of  the  iron. 

But  all  this  criticism  led  to  exhaustive  tests,  and  when  the 
professors  found  that  my  samples  stood  up  better  than  the  plain 
bars,  one  even  went  as  far  as  to  suggest  that  I  had  doctored  my 
samples.  This  led  me  to  twist  half  of  each  test  rod  only,  and  the 
superior  strength  of  the  cold  twisted  iron  was  finally  admitted, 
and  in  due  time,  when  steel  became  common,  even  better 
results  were  had  with  cold  twisted  steel.  Even  at  this  present 
time,  I  do  not  believe  that  the  increase  in  strength  due  to  the 


4  REINFORCED   CONCRETE  BUILDINGS 

twisting  has  been  accounted  for.  In  this  connection  I  call 
attention  to  an  interesting  fact  first  discovered  by  Professor 
Hesse,  and  that  is,  that  bars  tested  at  once  after  twisting  do 
not  give  as  good  results  as  those  tested  five  or  more  days 
after  twisting,  showing  that  a  certain  slow  change  takes  place 
in  the  structure  of  the  iron. 

From  the  earliest  time  of  my  career  I  have  experimented 
extensively  with  concrete  mixers  and  other  machinery,  and  the 
Ransome  mixer  is  now  a  standard  article.  However,  a  descrip- 
tion of  these  experiments  would  carry  us  too  far,  and  might 
not  interest  the  reader.  Suffice  it  to  say  that  my  first  patent 
for  a  concrete  mixer  was  granted  in  1884,  to  be  followed  by 
many  more. 

Up  to  about  1888  my  work  in  reinforced  concrete  was 
largely  confined  to  what  we  now  term  small  and  unimportant 
structures.  The  Bourn  &  Wise  wine-cellar  at  St.  Helena,  Cal., 
was  erected  in  1888;  the  building  is  75'  X  400',  three  stories 
high,  with  stone  walls.  The  main  floor  only  was  of  reinforced 
concrete  resting  upon  iron  columns.  The  design  is  shown  in 
Figure  3.  The  next  floors  were  erected  for  the  Californian 


FIGURE  3. 

Academy  of  Sciences  in  San  Francisco,  Figure  4;  during  con- 
struction these  floors  were  subject  to  much  adverse  criticism 
from  many  architects,  builders,  and  members  of  the  Society, 
and  efforts  were  made  to  have  the  fire  wardens  condemn  the 
work.  However,  inspectors  from  the  fire  department  found 
nothing  about  the  construction  that  could  be  injured  by  fire, 
and  having  sense  enough  to  perceive  its  great  strength,  they 
declined  to  take  any  action  in  the  matter.  To  satisfy  all  skep- 
tics in  regard  to  the  strength,  a  section  of  the  second  floor 
15'  X  22'  was  uniformly  loaded  with  gravel  to  415  Ibs.  per 
square  foot;  the  deflection  was  J".  For  the  further  satisfaction 
of  the  doubtful  the  load  was  left  on  for  four  weeks,  but  very  few 


PERSONAL  REMINISCENCE  5 

availed  themselves  of  invitations  to  examine  the  work  a  second 
time.     The  few  who  came  were,  however,  convinced. 

The  Leland  Stanford  Jr.  Museum,  at  Palo  Alto,  Cal.,  was 


FIGURE  4.  —  ACADEMY  OF  SCIENCES,  SAN  FRANCISCO 

Reinforced  Concrete  Floors,  Cast  Iron  Columns 

Erected  by  Ernest  L.  Ransome 

erected  about  this  same  time,  and  the  entire  wall  and  floor  con- 
struction was  of  concrete,  the  walls  having  superficial  joint  lines 
as  indicated  in  my  patent,  No.  405,  800,  of  1889  (Figure  144A). 
The  outside  surface  was  partly  tooled,  and  the  whole  was  built 


6 


REINFORCED  CONCRETE  BUILDINGS 


in  the  classical  design  originally  made  for  sand  stone.  The 
greatest  innovation  was,  however,  the  roof,  and  this  was  probably 
the  first  instance  on  record  where  a  finished  and  exposed  roof 
was  made  entirely  of  concrete.  The  roof  was  supported  on  iron 
trusses  10  ft.  on  centers  and  the  concrete  construction  rested 
upon  the  iron  rafters,  as  shown  in  Figure  5.  The  roof  over  the 
central  pavilion  is  quite  flat  and  is  46'  X  56'  in  plan,  reinforced 
with  2"  twisted  bars  60  ft.  .long  and  with  a  flat  reinforced  con- 
crete dome  panelled  with  I"  thick  glass.  In  the  same  location 
the  Girls  Dormitory  of  the  Stanford  University  was  erected  soon 


FIGURE  5. 

afterwards,  and  its  three  stories  were  completed  in  ninety  days 
from  the  time  the  plans  were  ordered. 

When,  on  April  18,  1906,  San  Francisco  was  destroyed  by  the 
earthquake,  the  buildings  at  Palo  Alto  suffered  severe  damage, 
in  many  cases  beyond  repairs.  However,  the  old  reinforced 
concrete  buildings  referred  to  above  stood  the  test  with  little 
if  any  damage;  see  Bulletin  No.  324  of  the  United  States  Geo- 
logical Survey,  pages  22,  23,  24,  75,  112-114. 

It  may  be  worth  while  to  note  that  the  addition  to  the  Borax 
Works  at  Alameda,  Cal.  (1889),  was  the  first  instance  of  the 
ribbed  floor  construction  erected;  it  will  be  seen  from  Figure  6 
that  the  construction  is  identically  the  same  as  used  for  that 
kind  of  floors  today.  The  Columns  were  also  of  concrete,  prob- 
ably the  first  ever  erected. 

I  desire  to  express  here  my  sincere  gratitude  to  the  men  who, 
in  those  early  times,  had  the  confidence  and  foresight  to  realize 


PERSONAL  REMINISCENCE  7 

the  technical  and  commercial  importance  of  the  novel  construc- 
tion, often  in  the  face  of  severe  criticism  and  bitter  attacks. 
Chief  amongst  these  are  my  associate  for  many  years,  Mr.  Frank 
M.  Smith,  Architect  Percy  and  Governor  Stanford,  deceased. 


FIGURE  6. 

Additional  information  in  regard  to  the  preceding  buildings 
may  be  found  in  a  paper  "  Concrete  Construe tion,"  read  by 
George  W.  Percy  before  the  San  Francisco  Chapter  of  the 
American  Institute  of  Architects,  February  9,  1894. 

In  this  paper  reference  is  also  made  to  my  tests  on  delaying 
the  placing  of  1:2  mortar;  the  results  are  really  astonishing. 
The  tensile  strength  of  briquettes  made  with  Knight,  Bevan, 
and  Sturgess  cement  was  as  follows: 

Delay  in  hours  0  1  2  2^          3£  4£ 

Tens.  Strength,  Ibs.  252        228        240        256        306        228 

showing  that  a  delay  of  2J  to  3£  hours  really  gave  the  highest 
results.  Similar  tests  with  White's  cement  gave  the  best  results 
with  a  delay  of  1J  hours,  after  which  the  strength  fell  rapidly. 
In  all  these  cases,  the  concrete  was  worked  up  again  as  soon  as 
it  stiffened.  Unfortunately,  these  tests  have  never  been  ex- 
tended to  modern  cement.  However,  in  my  address  to  the 
Society  of  American  Architects,  October  17,  1894,  I  called 
attention  to  the  truly  remarkable  results  obtained  by  Mr. 
Spencer  Newberry,  who  found  that  a  mixture  1  :  3  which,  when 
worked  for  one  minute  with  a  trowel,  developed  a  tensile  strength 
of  87  Ibs.  in  seven  days,  developed  a  strength  of  240  Ibs.  in  the 
same  period  after  being  worked  with  a  trowel  for  five  minutes. 
I  also  made  experiments  with  continued  mixing,  keeping  the 


8 


REINFORCED  CONCRETE  BUILDINGS 


concrete  in  the  mill  for  as  many  as  1000  revolutions.  I  found 
that  within  this  limit  the  strength  of  the  concrete  increased  with 
the  number  of  revolutions,  so  that  concrete  given  1000  turns  in 
the  mill  was  stronger  than  when  it  had  had  700  revolutions  only.1 
These  observations  led  me  to  believe  in  the  continued  work- 
ing of  the  concrete  while  it  is  setting;  that  is,  when  making  a 
slab,  I  put  men  on  with  rollers  who  were  instructed  to  keep  the 
rollers  going  for  several  hours.  The  slab  is  laid  on  the  forms  in 
the  usual  manner;  as  soon  as  the  concrete  is  hard  enough  to 
carry  a  man,  the  rolling  begins,  and  is  carried  on  with  two  or 


FIGURE  7. 

three  sets  of  rollers  of  increasing  weight  until  the  rollers  make 
no  impression.  A  more  handy  method  is  to  use  hollow  iron  drums 
filled  with  increasing  amounts  of  water.  It  is  important  that 
the  floors  be  not  allowed  to  dry  out  too  quickly,  for  which 


FIGURE  8. 

reason  they  may  be  sprinkled  if  necessary  during  rolling  and  kept 
moderately  wet  for  at  least  one  week  more. 

The  construction  of  " illuminating  panels"  in  concrete  floors, 
or,  as  they  are  more  commonly  called,  sidewalk  lights,  has  cap- 
tured the  attention  of  inventors  for  many  years.  But  owing  to 
the  unequal  expansion  of  glass  and  iron,  the  great  majority  of 
such  constructions  embodying  a  combination  of  these  two 
elements  have  not  been  satisfactory.  My  patents,  No.  448,993 
(1891),  Figure  7,  and  518,045  (1894),  Figure  8,  aimed  to  avoid 

1  It  must  here  be  noted  that  the  mill  used  for  this  experiment  was  of  a 
different  type  from  those  used  today,  the  modern  machines  having  a  much 
more  severe  action.  The  danger  in  overmixing  is  that  the  aggregate  is  ground 
very  fine,  thus  giving  a  mortar  with  an  excess  of  sand. 


PERSONAL  REMINISCENCE 


9 


the  use  of  the  iron  plate  by  setting  the  glasses  in  a  body  of 
reinforced  concrete,  and  this  was  accomplished  with  great  suc- 
cess. The  Ransome  Sidewalk  Lights  may  be  seen  in  every 
large  city  of  the  country,  amongst  other  places  the  New  York 
Subway,  and  my  patents  formed  during  their  terms  the  basis 
for  a  large  and  prosperous  industry. 

One  of  the  problems  most  troublesome  to  the  reinforced  con- 
crete engineer  is  encountered  in  joining  new  concrete  to  old. 
A  more  or  less  suitable  joint  may  be  had  in  a  number  of  ways, 
and  from  time  to  time  I  have  given  this  problem  much  thought. 
A  purely  mechanical  bond  is  created  by  bedding  an  open  coil 
half  way  in  the  old  concrete  surface,  so  that  the  other  half  is 
caught  in  the  new  concrete,  subsequently  molded  against  the  old 
surface,  Patent  No.  647,904  (Figure  9).  This  principle  is 


.A\\A\A\A\A\A\ 

wwwwwwfwwwwww 


FIGURE  9. 

utilized  in  the  "  unit "  construction  of  reinforced  concrete 
buildings  according  to  my  patent  No.  694,577  (1902),  Figure  10, 
and  the  tie  thus  made  is  so  efficient  that  the  subsequently 


0 

n°       ° 

^^VTTTVx 

° 

1 

FIGURE  10. 

molded  slab  may  serve  as  the  compression  flange  for  the  beam 
or  girder  erected  ahead  of  the  slab.  The  first  application  of  this 
principle  was  in  connection  with  the  office  building  erected  for 
the  Foster-Armstrong  Company  at  East  Rochester,  N.  Y. 
(1904-5),  and  it  has  since  been  extensively  used.  Honey- 
comb slag  may  also  be  utilized  in  a  similar  manner,  Patent 
No.  694,578  (1902),  especially  for  large  surfaces,  but  here  I 


10  REINFORCED  CONCRETE  BUILDINGS 

prefer  my  later  invention,  the  removal  of  the  surface  skin  with 
hydrochloric  acid.  The  surface  is  next  washed  with  water, 
and  the  finish  coat  is  then  placed  in  the  usual  manner,  Patent 
No.  800,942  (1905). 

This  latter  method,  "  the  acid  joint,"  has  been  tried  in  prac- 
tice with  excellent  results.  Fresh  bases  were  welded  to  old 
concrete  cylinders,  the  mass  allowed  to  set,  and  in  all  cases  the 
concrete  would  split  apart  from  the  joint  when  tested.  One  of 
my  superintendents  found  himself  unable  to  believe  in  the 
superiority  of  the  new  joint,  and  I  had  therefore  a  slab,  about 
4'  X  4',  set  aside,  letting  him  finish  one  half  by  any  method 
desired,  and  reserving  the  other  half  for  my  acid  joint.  Try 
as  he  would,  he  never  succeeded  in  making  an  unbreakable  * 
joint.  A  reward  of  ten  dollars  was  promised  to  any  man  on  the 
job  who  could  separate  the  finish  from  the  base  on  the  half 
treated  with  acid,  and  while  many  of  the  men  availed  themselves 
of  the  opportunity,  the  reward  is  as  yet  unearned. 

The  Pacific  Coast  Borax  Co/s  building,  at  Bayonne,  N.  J., 
erected  in  1897-98,  in  a  measure  marks  the  closing  of  the  old- 
time  construction  of  reinforced  concrete  buildings,  constructed 
more  or  less  in  imitation  of  brick  or  stone  buildings,  with  com- 
paratively small  windows  set  in  walls  (Figure  11).  This  build- 
ing, however,  occasioned  the  discovery  of  an  important  fact, 
that  of  the  greatly  improved  fire-resistance  of  concrete  mixed 
with  salt.  Before  that  time,  salt  had  been  known  and  used  as  a 
frost  preventative,  and  as  this  building  was  constructed  in  the 
winter,  I  desired  to  use  salt.  I  had  some  doubts  as  to  the 
strength  of  concrete  so  made,  and  I  also  anticipated  some 
trouble  with  efflorescence.  A  number  of  test  cubes  were  made 
with  salt,  some  mixed  by  hand,  others  by  mill,  and  to  my  sur- 
prise I  found  that  the  hand-mixed  specimens  showed  efflorescence 
while  the  machine-mixed  specimens  did  not.  I  am  .unable  to 
explain  this  difference.  As  to  the  strength,  I  found  it  was  not 
impaired  by  the  salt,  when  salt  to  the  extent  of  one  to  five  per 
cent,  of  the  weight  of  the  cement  was  added;  I  also  found  that 
the  specimens  without  salt  showed  air  or  hair  cracks,  while  those 
with  salt  did  not.  It  now  occurred  to  me  that  salt  might  have 
the  same  or  similar  effect  on  concrete  that  it  has  on  clay;  it 
is  well  known  how  clay  pipes,  etc.,  are  glazed  by  being  burned 
with  salt.  Test  cubes  with  and  without  salt  were  heated  in 


g 


Cfi       03    "S 


6J1 

g     o   g 
fe     w    S 

l?fl 

I  I 

1-H       'S 


«         -H 

g     I 


12  l    tifi  IN  FORCED  CONCRETE  BUILDINGS 

the  boiler  furnace  to  a  red  heat,  and  then  plunged  into  cold 
water;  the  specimens  without  salt  had  a  soft  surface  easily 
picked  to  pieces  with  the  bare  fingers,  while  those  with  salt  were 
intact,  and  the  compressive  strength  appeared  to  be  unchanged. 
I  had  then  the  belief  that  different  brands  of  cement  were  affected 
in  different  ways  by  the  addition  of  salt,  but  I  have  so  far  never 
found  a  Portland  Cement  that  was  injured  in  the  least  by  addi- 
tion of  the  quantities  indicated. 

Of  other  additions  to  Portland  Cement  with  which  I  have 
experimented  I  must  mention  lime  and  clay.  The  former 
addition  is  so  liable  to  abuse  that  I  have  largely  abandoned  it, 
except  for  the  construction  of  waterproof  tanks,  and  even  then 
it  is  not  indispensable.  The  fact  seems  to  be  that  an  addition 
of  from 'three  to  five  per  cent,  of  slacked  lime  is  beneficial  when 
added  as  "  milk  of  lime,"  using  the  limey  water  for  mixing 
instead  of  plain  water;  the  trouble  arises  as  soon  as  lumps  of 
lime  putty,  however  small,  find  their  way  into  the  concrete,  or 
when  the  amount  exceeds  five  per  cent.  In  cases  where  the 
concrete  for  this  or  other  reasons  contains  free  lime,  I  have 
sometimes  hastened  the  setting  by  giving  an  artificial  supply 
of  carbonic  acid;  usually  supplying  heat  to  the  concrete  at  the 
same  time.  For  the  details  of  this  see  my  patent  No.  652,732 
(1900). 

As  to  an  addition  of  clay  I  have  found  that  from  two 
to  three  per  cent,  in  the  aggregate  are  beneficial  rather  than 
detrimental,  and  such  moderate  amounts  help  greatly  to  render 
the  concrete  waterproof.  My  attention  was  first  called  to  this 
matter  when  conditions  compelled  me  to  use  Niagara  Gravel 
for  some  works  in  Buffalo  in  about  1892,  and  I  declined  at  first 
to  use  this  material  as  it  evidently  contained  considerable  quan- 
tities of  clay.  Inspection  of  concrete  work  made  by  other 
parties  soon  convinced  me  that  the  gravel  in  question  was 
excellent  material,  and  earlier  tests  by  Mr.  Clarke  of  Boston 
fully  corroborated  my  own  observations  on  this  point.  I  do 
not  wish  to  go  on  record  as  stating  that  all  clay  is  beneficial, 
and  if  the  grains  of  sand  are  coated  with  a  film  of  clay,  I  am 
convinced  that  it  has  a- most  dangerous  effect. 

In  the  years  between  1900  and  1902  I  developed  a  radical 
departure  in  the  exterior  construction  of  reinforced  concrete 
factory  buildings,  consisting  mainly  in  the  extension  of  the  floor 


PERSONAL  REMINISCENCE 


13 


plate  or  slab  over  the  exterior  columns,  forming  a  belt  course 
on  the  outside  of  the  building.  Between  the  exterior  piers, 
upward  and  downward  extensions  were  added;  the  former  to 
be  added  after  the  next  floor  had  been  constructed,  and  the 
latter  forming  an  integral  portion  of  the  floor  proper.  This 
innovation  forms  the  subject  matter  of  my  patent  No.  694,580 
(1902),  Figure  12,  according  to  which  a  very  large  number  of 
buildings  have  been  erected  throughout  the  country. 

The  principal  advantages  of  this  construction  as  compared 


-£- 


FIGURE  12. 

with  the  old  solid  concrete  walls  are,  aside  from  the  purely 
technical  ones,  that  large  window  areas  are  easily  made  possible, 
that  the  curtain  walls  are  utilized  as  carrying  members,  and  the 
shrinkage  of  the  walls  is  taken  care  of  by  means  of  the  expansion 
joints  existing  at  each  end  of  each  curtain  wall,  the  same  being 
recessed  into  the  sides  of  the  piers.  That  this  construction 
affords  great  economy  will  be  evident  from  the  fact  that  all  the 
curtain  walls  may  be  cast  with  a  few  forms  only,  the  several 
forms  being  usually  removed  in  twenty-four  to  forty-eight 
hours  after  pouring. 

The  first  building  erected  under  this  patent  was  the  Kelly  & 
Jones  Co.'s  machine  shop  at  Greenburg,  Pa.,  60'  X  300',  four 
stories  high  (1903-4),  Figure  13,  built  by  the  Ransome  & 


14*  *    REINFORCED   CONCRETE  BUILDINGS 


PERSONAL  REMINISCENCE 


15 


Smith  Co.,  and  followed  by  many  more,  chief  amongst  which  I 
mention  the  machine  shop  for  the  United  Shoe  Machinery  Co., 
at  Beverly,  Mass..,  aggregating  about  sixteen  acres  of  floor  space 
(the  recent  additions  comprising  about  four  acres  were  built 
under  the  Unit  System),  and  the  Foster-Armstrong  Go's  Piano 
Factory  at  East  Rochester,  N.  Y.,  including  a  dozen  or  more 
large  buildings. 

From  this  time  also  dates  my  invention  of  the  coil  joint  for 
uniting  reinforcing  bars,  consisting  in  an  open  metallic  coil 
surrounding  the  lapping  ends  of  the  bars  to  be  joined;  No. 
694,576  (1902),  Figure  14.  It  is  particularly  well  adapted  for 
deformed  and  especially  twisted  bars,  because  the 
initial  sliding  of  the  bars  cannot  take  place  with- 
out driving  out  a  wedge  of  the  surrounding  con- 
crete, and  this  is  effectively  prevented  by  the 
coil.  Wherever  beams  have  been  built  with  bars 
joined  on  this  principle  the  results  have  been  sat- 
isfactory, and  the  much  later  tests  by  Professor 
Morsch  fully  substantiate  everything  claimed  for 
the  coil  joint.  (See  Trautwine:  Concrete,  1909, 
p.  1174,  where  plain  bars  only  have  been  used.) 

During  all  this  time  I  have  given  consider- 
able study  to  the  proper  design  of  the  falsework, 
realizing  in  common  with  other  concrete  men  that 
the  handling  of  the  forms  in  many  cases  meant 
the  difference  between  loss  and  profit.  The  stand- 
ardization of  the  forms  and  their  repeated  use 
is  one  way  of  approaching  this  problem,  and 
with  a  standard  layout  there  is  no  question  but  that  invest- 
ment in  molds  of  a  more  permanent  nature  is  a  paying  propo- 
sition. More  information  in  regard  to  this  may  be  found  in  .a 
later  chapter  on  forms;  suffice  it  here  to  say  that  I  have 
made  "  coreboxes "  for  one  building  and  used  them  there 
four  times,  shipped  them  to  another  building  and  used  them 
seven  times,  then  again  shipped  them  and  used  them  four 
times,  and  finally  shipped  them  once  more  and  used  them, 
but  on  the  last  job  the  repairs  were  so  expensive  that  the  profit 
was  doubtful.  I  am  now  convinced  that  the  final  solution  of 
the  questions  pertaining  to  economical  construction  must  be 
found  along  other  lines,  and  I  have  had  sufficient  experience 


FIGURE  14. 


16  REINFORCED  CONCRETE  BUILDINGS 

with  "  Unit  Construction  "  to  warrant  the  statement  that  great 
economy  and  better  workmanship,  as  well  as  quicker  work,  is 
thus  obtained.  From  a  first  attempt  in  1905,  and  subsequent 
experience,  I  have  evolved  a  system  adapted  to  buildings  with 
many  stories,  known  as  "  The  Ransome  System  of  Unit  Construc- 
tion," which  has  been  extensively  used  and  is  now  being  used 
on  work  of  considerable  dimensions.  (Patents  No.  694,577, 
1902,  and  918,699,  1909.) 

Not  very  long  ago,  a  patent  was  granted  for  "  wet  mixed  " 
concrete  to  a  Western  gentleman,  and  this  brings  back  to  memory 
the  historical  fact  that  wet  mixture  has  been  known  from  the 
earliest  days  of  the  art.  Thus,  Coignet's  early  patents  (1869) 
speak  of  it  and  recommend  it,  but,  nevertheless,  dry  concrete 
rammed  in  was  in  general  favor.  Now,  one  of  the  products  of 
the  old  Ransome  Stone  Company  was  porous  filter  stones,  made 
under  the  old  process,  and  I  was  very  much  interested  in  making 
similar  stones  of  Portland  Cement  concrete.  Owing  to  the  lack 
of  uniformity  of  the  concrete  stones,  I  never  made  a  success  of 
this,  but  I  did  find  that  in  order  to  make  the  concrete  sufficiently 
porous  for  the  purpose,  I  had  to  use  a  dry  mix.  Reversely,  in 
making  ornamental  stones,  I  always  had  better  results  with  wet 
mixtures,  especially  for  the  facing. 

It  is  believed  that  the  argument  in  favor  of  the  dry  mix  was 
based  upon  the  fact,  known  as  early  as  1890,  that  dry  mixed 
mortar  rammed  hard  into  the  briquette  molds  gave  higher 
strength  in  the  tensile  tests  than  wet  mixture.  The  arching 
effect  of  the  stone  in  the  concrete  was  disregarded.  Personally 
I  was  confirmed  in  my  observations  by  Bamber's  tests  which 
ingeniously  proved  the  fallacy  of  the  arguments  in  favor  of  the 
dry  mix,  and  showed  the  greater  density  of  a  wet  mix:  A  dry 
batch  was  made,  and  rammed  thoroughly  into  a  mold  2'  X  2'  X  2' 
so  as  to  fill  it  level  full.  The  mixing  platform  was  now  cleaned, 
and  the  contents  of  the  box  dumped  out  on  the  mixing  board, 
thoroughly  remixed  with  enough  water  to  make  a  "  wet  "  mix, 
and  then  replaced  in  the  box.  But  it  now  proved  that  the  box 
lacked  2"  in  being  full,  so  that  the  greater  compactness  or  density 
of  the  wet  mix  was  proved.  Other  tests  have  proved  the  superior 
strength  of  wet  concrete,  and  that  the  densest  mixture  is  also  as  a 
rule  the  strongest;  as  to  the  permanency  I  have  had  occasion 
to  compare  dry  and  wet  mixed  concrete  after  they  had  been  in 


PERSONAL  REMINISCENCE  17 

place  for  many  years,  and  found  the  wet  mixture  much  harder 
than  the  dry  placed  concrete. 

From  my  long  and  varied  experience  with  concrete  I  desire 
to  state  that  I  have  found  no  agency  which  actually  injured  old 
well-made  concrete  properly  proportioned,  except  acid.  Such 
items  as  sewage  and  oils  have  had  no  influence,  neither  have  I 
found  that  the  gases  from  the  salamanders  injure  the  setting 
concrete. 

In  closing  this  contribution  to  the  history  of  Reinforced  Con- 
crete, I  cannot  help  but  marvel  at  the  enormous  growth  of  the 
concrete  industry  during  the  last  fifty  years,  and  especially  of 
the  reinforced  concrete  industry  in  its  less  than  thirty  years  of 
actual  use.  I  venture  to  predict  that  the  next  thirty  years  will 
see  even  greater  advancements,  but  I  would  also  ask  the  younger 
men  in  the  profession  to  remember  that  real  knowledge  and  ever- 
lasting care  are  necessary,  so  that  the  reinforced  concrete  indus- 
try in  the  future  may  proceed  without  setbacks  from  accidents 
caused  by  neglect  or  greed. 


CHAPTER   II 

BASIC  PATENTS  FOR  INVENTIONS  RELATING  TO  REINFORCED 
CONCRETE,  AND  A  SHORT  SURVEY  OF  THE  EARLY  HIS- 
TORY OF  THE  ART 

BY  ALEXIS  SAURBREY 

REINFORCED  concrete  as  used  today  may  be  said  to  have 
arisen  from  the  following  basic  inventions:  (1)  A  combination 
of  a  plastic  material  adapted  to  harden,  with  a  metallic  strength- 
ening device,  the  word  "  plastic  "  being  here  used  in  its  widest 
sense  so  as  to  include  masonry  laid  with  a  plastic  mortar.  (2) 
That,  in  a  combination  of  this  kind,  the  concrete,  or  plastic 
material,  must  carry  whatever  compressive  stresses  act  in  the 
structure,  and  the  metal  the  tensile  stresses.  (3)  That  there- 
fore the  concrete  and  the  steel  have  a  tendency  to  separate,  so 
that  "bond"  or  "anchorage"  must  be  provided,  either  locally 
in  certain  parts  of  the  structure,  or  continuously  along  the  length 
of  the  metal  reinforcement.  (4)  That,  in  addition  to  a  main, 
or  directly  tensile  reinforcement,  a  secondary,  transverse  rein- 
forcement is  desirable  and  beneficial,  whether  this  transverse 
reinforcement  is  made  from  separate  bars,  or  some  of  the  main 
bars  are  arranged  in  a  peculiar  manner  to  gain  the  desired  effect. 
(5)  That  a  compression  member  may  be  strengthened  by  longi- 
tudinal as  well  as  by  transverse  reinforcement,  or  both.  (6) 
That  a  multitude  of  various  uses  may  be  found  for  the  compound 
material,  each  requiring  a  special  combination. 

To  trace  back  into  remote  antiquity  the  use  of  metal  in 
combination  with  brick  work  is  beyond  the  scope  of  this  book; 
suffice  it  to  say  that  the  Romans  are  sometimes  credited  with 
the  first  use  of  such  constructions:  it  is  said  that  a  tomb  has 
been  found  in  which  the  roof  consisted  of  a  concrete  slab  with 
bronze  rods  embedded,  crossing  each  other  lattice-wise.  This 
construction  dates  a  hundred  years  or  more  B.C.1  It  appears 

1  "Reinforced  Concrete,"  compiled  by  James  Tozer  &  Son,  Limited, 
Birkenhead,  England. 

18 


BASIC  PATENTS  FOR  INVENTIONS  19 

that  on  the  authority  of  ancient  writers,  the  prevailing  method 
of  judging  the  quality  of  lime  for  setting  purposes  was  by  observ- 
ing the  hardness  and  color  of  the  original  stone,  the  harder  and 
whiter  varieties  being  preferred,  and  that  this  method  was  in 
general  use  for  a  score  of  centuries  or  more,  until  the  more 
modern  method  of  learning  by  experiment  and  investigation  of 
the  facts  was  first  applied  to  .the  subject  by  Smeaton  in  England, 
in  or  soon  after  the  year  1756.  Smeaton  is  credited  with  the 
discovery,  as  a  result  of  actual  chemical  analysis,  that  the  real 
cause  of  the  setting  of  limes  and  cements  consisted  in  a  combina- 
tion of  clay  with  lime. 

The  use  of  the  English  natural  cement,  commonly  called 
"  Roman  Cement,"  was  discovered  by  Parker  in  1796,  who  in 
that  year  took  out  a  British  patent,  No.  2120,  for  a  cement 
or  tarrass  to  be  used  in  aquatic  or  other  buildings  and  stucco 
work.  The  use  of  the  word  "Portland  Cement"  first  occurs  in 
the  specification  of  a  patent  granted  in  1824  to  Joseph  Aspdin, 
of  Leeds,  England,  No.  5022,  owing  its  name  to  its  resemblance 
to  Portland  Stone,  and  this  discovery  formed  the  basis  of  con- 
siderable manufacturing  operations  after  the  establishment  of  a 
factory  at  Wakefield  in  1825. 

The  Period  of  Discovery.  Although  in  1847  three  or  four 
cement  mills  manufacturing  artificial  cement  were  operating 
in  England,  the  use  of  the  new  product  was  limited,  until  definite 
methods  of  determining  the  commercial  value  of  the  product  were 
developed.  Amongst  the  engineers  who  made  reliable  and 
scientific  observations  in  this  field,  John  Grant,  subsequently 
knighted  for  his  eminent  ability  as  engineer,  must  be  mentioned 
in  the  first  line,  as  well  as  General  C.  W.  Pasley,  whose  book  on 
" Limes  and  Calcareous  Cements"  was  a  standard  work  in  its 
day  (first  edition  in  1838,  second  in  1847).  From  this  work  we 
learn  how  the  relative  merits  of  the  various  cements  were  some- 
times tested  by  building  a  row  of  bricks  out  from  the  face  of  a 
wall  (Figure  15),  as  many  as  twenty-nine  or  thirty  bricks  having 
been  stuck  out  in  this  manner  in  one  day,  and  thirty-three  bricks 
in  thirty-three  days,  before  the  bricks  fell.  The  famous  semi- 
arches  constructed  by  Sir  M.  J.  Brunei,  the  builder  of  the  Thames 
Tunnel,  were  undoubtedly  conceived  in  this  same  spirit,  and 
these  arches  are  all  the  more  interesting  as  they  give  the  first 
known  rational  application  of  the  principle  of  strengthening 


20 


REINFORCED  CONCRETE  BUILDINGS 


masonry  by  means  of  tension  rods.  The  arches  are  shown  in 
Figure  16;  the  work  was  four  years  in  building,  having  been 
added  to  from  time  to  time,  and  it  fell  on  January  31,  1838,  as  a 
result  of  a  cave-in  in  an  adjoining  excavation.  The  long  arch 


flow  of  Bricks  standing  out  flat 


T?ow  of  Bricks  standing  out  en  edge 


FIGURE  15. 

was  60  ft.,  and  the  other  about  37  ft.  long,  the  latter  being 
loaded  at  its  extremity  with  a  weight  of  62,700  Ibs.,  and  this 
remarkable  result  was  obtained  by  the  introduction  of  wooden 
lath  and  hoop-iron  bonding  strips  inserted  in  the  joints  of  the 
brick  work.  "  This  ingenious  arrangement  of  Mr.  Brunei  will 


FIGURE  16. 

probably  be  found  hereafter  of  great  value  in  practical  archi- 
tecture," Pasley  says,  and  time  has  shown  that  he  was  correct 
in  his  prediction. 

Brunei  also  built  a  brick  beam  25'  1"  long,  reinforced  with 
strips  of  hoop-iron;  this  beam  was  broken  in  1836  under  a  load 
of  27,025  Ibs.  A  similar  experiment  was  made  at  the  Francis 
Cement  Factory  at  Vauxhall,  the  dimensions  of  the  brick  beam 
being  4'  9"  deep  by  22J"  wide,  reinforced  with  fifteen  pieces  of 
1J"  hoop-iron  in  the  bottom  portion.  The  span  was  21'  4"  in 
the  clear,  and  the  beam  was  broken  under  a  load  of  50,652  Ibs. 

Pasley  had  now  several  such  beams  made,  one  laid  in  neat 
cement  without  irons,  one  exactly  similar,  but  reinforced  with 
five  longitudinal  irons,  and  a  third  beam  similar  to  the  second 
one,  but  laid  in  1  :  3  lime  mortar.  The  result  showed  the  su- 


BASIC  PATENTS  FOR  INVENTIONS 


21 


periority  of  the  reinforced  beam  laid  in  cement,  for  the  first  beam 
carried  only  498  Ibs.,  while  the  second  carried  4523  Ibs.,  and  the 
third  failed  by  sliding  of  the  irons  under  a  load  of  only  742  Ibs. 

As  early  as  1832,  Ranger  took  out  a  British  patent,  No.  6341, 
for  making  certain  kinds  of  mortar  with  hot  water,  and  this 
invention  was  used  in  the  first  known  case  of  modern  engineering 
work  of  any  consequence  executed  entirely  in  concrete,  viz.,  a 
dock  at  Woolwich  dockyard  (1835)  and  sea-walls  at  Woolwich 
and  Chatham.  The  floor  of  the  dock  was  a  failure,  but  the  sea- 
wall at  Woolwich  was  standing  in  perfect  condition  in  1879. 
" Ranger's  artificial  stone"  became  well  known  in  England,  but 
it  was  soon  discovered  that  cold  water  was  quite  sufficient  for 
making  good  concrete. 

A  short  list  of  some  early  English  patents  may  be  of  interest: 
Wilkinson,  1854,  No.  2293  (Figure  17). 


FIGURE  17. 

"The  wire  rope  H, H,  is  secured  at  its  extremities  at  each 
line  of  support  by  imbedding  it  in  the  mixture  or  concrete  while 
in  a  soft  state,  and  forming  the  ends  into  loops,  or  by  opening 
out  the  strands  and  hirling  them  in  various  directions,  which 
renders  it  so  secure  as  not  to  be  drawn  out  under  any  force  short 
of  the  breaking  weight  of  the  rope.  For  ordinary  dwelling 
houses  I  propose  placing  such  wire  ropes  about  nine  inches  apart, 
and  to  have  a  full  depth  of  floor  of  one-sixteenth  the  span." 
Dennet,  1857,  No.  685  (Figure  18). 


FIGURE  18. 

Proposes  to  strengthen  his  arches  with  lamina  of  wood  or 
iron. 
Bunnet,  1858,  No.  1292  (Figure  19). 

Uses  iron  tie  rods  and  metal  abutment  plates  for  his  arches 
of  hollow  blocks. 


22 


REINFORCED  CONCRETE  BUILDINGS 


Parkes,  1863,  No.  317. 

Proposes  an  iron  bond  consisting  of  a  band  or  strip  of  iron 
with  transverse  teeth,  ridges,  ribs  or  projections  pressed  out  of 
the  solid,  raised  at  intervals  on  each  side  of  the  strip  for  the  whole 
width  thereof.  He  employs  two  rollers,  with  suitable  indenta- 
tions, for  the  manufacture. 


FIGURE  19. 

Ransome  (Fk.)  1865,  No.  1337. 

Molds  slabs  of  artificial  stone  around  pieces  of  hoop-iron  on 
edge  running  from  end  to  end,  so  that  the  hard  concrete  prevents 
the  irons  from  buckling  under  load. 
Scott,  1867,  No.  452  (Figure  20). 


1 

v////////////s///////////////j/}?/7y/////////////tf  ///>/£•/'/.>*/////>///»////. 

^s 

1 

IT          TT 

FIGURE  20. 

" 

1 

Proposes  to  dispense  with  the  use  of  ordinary  joists  and  to 
make  use  of  wrought-iron  tie-rods  extending  from  wall  to  wall. 
"The  floor  becomes  one  solid  beam, having  the  tie-rods  and  hoop- 
iron  in  combination  with  the  concrete  to  take  the  tensile  strain, 
and  the  concrete  to  take  the  compressive  action  resulting  from 
the  weight  of  the  floor." 
Lythgoe  &  Thornton,  1868,  No.  640  (Figure  21). 


FIGURE  21. 

"  The  method  of  constructing  floors  with  bars  of  J_-iron  and 
concrete  as  shown." 
Johnson  (Coignet)  1869,  No.  884  (Figure  22). 

An  invention  relating  to  the  facing  of  concrete  blocks  with 
cast-iron  or  steel  protecting  plates,  to  be  used  as  street  curbs, 
etc. 
Gedge  (Monier)  1870,  No.  1999  (Figure  23). 


BASIC  PATENTS  FOR  INVENTIONS 


23 


"  In  short    the  iron    is    the   skeleton   and  the   cement  its 
covering." 
Tall,  1871,  No.  1001  (Figure  24). 

Iron  hooping,  wirework,  or  netting  are  interlocked  between 


FIGURE  22. 


FIGURE  23. 


the  lateral  cross  bars,  and  form  a  close  lattice  or  basketwork. 
Portland  Cement  stucco  is  applied. 
Brannon,  1871,  No.  2703  (Figure  25). 


FIGURE  24. 

"  Wirework  embedded  in  concrete,  to  give  cohesive  strength 
against  transverse  and  tensile  strains." 
Hyatt,  1871,  No.  3124  (Figure  26). 


24 


REINFORCED  CONCRETE  BUILDINGS 


"  The  peculiar  construction  of  floor  which  I  designate  an 
'all-beam'  floor,  composed  of  a  number  of  separate  tubes  laid 
side  by  side." 
Turner,  1872,  No.  1396. 

On  the  iron  beams  "I  strain  my  wire  from  the  plates  in  the 
walls;  these  wires  are  intended  to  supersede  the  use  of  floor 
joists  of  wood,  and  will  form  beds  for  my  concrete  floors,  and  also 
answer  on  the  underside  instead  of  laths  for  the  plastered  ceil- 
ings, which  work  of  plastering  may  be  carried  on  at  the  same 
time  as  the  laying  on  of  the  floors  in  concrete." 
Emmens,  1872,  No.  2451  (Figure  27). 


FIGURE  25. 


FIGURE  27. 


"  The  employment  of  sheets  of  corrugated  iron  as  founda- 
tion for  roadways,  paths,  steps,  and  flooring." 
Lish,  1873,  No.  1621  (Figures  28,  29). 

The  drawing  shows  a  sectional  view  of  a  floor  and  girder 
of  concrete  with  tension  .rods  embedded  therein,  as  indicated  by 
the  dotted  lines. 
Hyatt,  1873,  No.  3684. 

Asbestos  combined  with  perforated,  corrugated  sheet  metal 
or  with  crimped  sheet  metal  or  upon  a  hollow  grate  bar  system. 
Coddington,  1873,  No.  1004  (Figure  30). 


BASIC  PATENTS  FOR  INVENTIONS 


25 


The  figure  shows  a  water  pipe  or  tube,  C  being  the  cemented 
material,  E  the  interwoven  metal. 


FIGURE  28. 


FIGURE  29. 


Hyatt,  1873,  No.  3381. 

"  The   system  or   mode  of   forming   cellular   or   honeycomb 
structures  by  connecting  together  single  cell  blocks  by  means 


FIGURE  30. 

of  tie-rods  or  crimped  blades  of  metal,  with  or  without  addi- 
tional straight  tie-rods." 


FIGURE  31. 


Hyatt,  1874,  No.  2550  (Figure  31). 

"I  form  the  tie  in  a  way  which  gives  it  power  to  grip  and 
hold  the  foreign  material  in  a  manner  and  by  a  method  which 


26 


REINFORCED  CONCRETE  BUILDINGS 


brings  the  load  and  consequent  strain  upon  the  tie  at  the  same 
instant  it  is  felt  by  the  concrete  or  foreign  material,  by  which 
means  the  tensile  and  compressive  forces  act  in  harmony  with 
each  other." 
Hyatt,  1874,  No.  1715  (Figure  32). 


FIGURE  32. 

"  Making  hollow  metal  beams  of  interlaced  lattice  or  open- 
work, as  the  holder  of  a  tie-rod,  to  connect  the  same  with  con- 
crete or  equivalent  material." 
Edwards,  1891,  No.  2941  (Figure  33). 

1892,  No.  1415  (Figure  34). 

1894,  No.  15,466  (Figure  35). 


L 


FIGURE  33. 


i.--. 


FIGURE  34. 

Edwards'  patents  show  a  remarkable  insight  into  the  nature 
of   reinforced   concrete   construction.     It   is   proposed   to   cast 


BASIC  PATENTS   FOR  INVENTIONS 


27 


the  slabs  separately  and  set  them  when  hard,  owing  to  the  great 
cost  of  the  centering;  the  bending  up  of  the  principal  tension 
rods  is  described  at  great  length,  and  stress  is  laid  upon  the 
benefit  of  many  small  rather  than  fewer  but  larger  rods.  The 


FIGURE  35. 

importance  of  preventing  sliding  of  the  reinforcement  is  shown, 
and  it  is  described  how  the  beams  may  be  pierced  by  openings 
in  much  the  same  manner  as  done  under  the  Visintini  System. 
The  benefits  as  well  as  troubles  arising  from  the  fixing  of  the 
ends  of  the  beams  into  the  walls  are  perfectly  understood,  and  the 
entire  argument  advanced  is  illustrated  by  tests  (by  Kirkaldy). 

While  in  England  the  new  construction  made  but  scant 
headway,  a  considerable  activity  took  place  in  Germany,  where 
the  Monier  Patents  were  bought  and  exploited  by  G.  A.  Wayss, 
and  where  M.  Koenen  advanced  the  first  rational  method  of 
calculation  in  1886.  The  " straight  line  formula"  was  fully 
discussed  by  Koenen  in  "  Centralblatt  der  Bauvervaltung," 
May  14,  1902,  and  is  to  this  day  the  commonly  accepted 
standard. 

In  Holland,  the  first  ribbed  floors  were  erected  in  1886  in 
connection  with  the  Public  Library  in  Amsterdam. 

In  France,  it  seems  that  Monier's  first  patent  was  taken  out 
in  1867,  but  it  has  been  intimated  that  he  had  knowledge  of 
the  earlier  patent  granted  to  Lambot,  who  had  made  a  reinforced 
concrete  boat  of  small  dimensions  in  1855.  This  boat  is  said 
to  be  in  existence  today.  Monier 's  efforts  toward  the  intro- 
duction of  his  inventions  were  not  very  successful,  partly  per- 
haps because  he  failed  to  realize  the  necessity  of  placing  the 
reinforcement  near  the  bottom;  it  is  told  that  when  Wayss 
showed  him  slabs  so  reinforced,  Monier  severely  criticized  this 
arrangement,  and  abruptly  ended  the  argument  by  exclaiming, 
"Who  is  the  inventor,  you  or  I?"1  As  a  matter  of  fact,  little 

1  Suenson:  Jaernbeton,  P.  5. 


28 


REINFORCED   CONCRETE  BUILDINGS 


was  done  in  building  construction  until  1892,  when  Henne- 
bique  and  Coignet  took  the  reinforced  concrete  construction 
•up  with  great  success,  each  introducing  his  own  system. 

In  the  United  States,  the  first  indication  of  anything  ap- 
proaching reinforced  concrete  may  be  found  in  a  patent  granted 
to  P.  Summer,  1844,  No.  3566  (Figure  36),  for  a  metal  lathing, 
which  was  still  further  improved  by  J.  B.  Cornell  in  1859,  No. 
22,939  (Figure  37).  At  this  early  date,  a  number  of  patents 


FIGURE  36. 

for  cement  pipes  were  granted,  as  to  R.  B.  Stevenson,  1854, 
No.  11,814,  for  a  combination  of  a  pipe  of  sheet-metal  and  an 
exterior  coating  of  hydraulic-cement  mortar  of  "  requisite  thick- 
ness for  strength."  In  the  Wyckoff  patent,  No.  32,100,  of  1861, 
the  interior  pipe  is  of  wood  wound  with  wire  of  iron  or  other 
metal;  in  the  Knight  Patent,  No.  32,298,  of  the  same  year,  a  metal 
tube  is  disposed  "  intermediate  between  the  inner  and  outer 
surfaces"  of  a  cement  pipe.  In  1868,  A.  P.  Stephens  took  out  a 
patent,  No.  78,336,  on  a  similar  pipe,  in  which  the  strengthening 
tube  was  made  of  corrugated  iron;  in  1872,  Patent  No.  127,438, 
the  tube  was  changed  to  a  spirally  formed  sheet  metal  tube, 
and  in  the  same  year  J.  A.  Middleton,  Patent  No.  133,875,  pro- 
posed to  strengthen  his  cement  pipes  by  a  layer  of  wirecloth 
embedded  in  the  cement,  thus  combining  what  we  now  consider 
the  essential  elements  of  a  reinforced  concrete  pipe  (Figure  38). 
The  first  reinforced  concrete  wall-patent  appears  to  be  one 


BASIC  PATENTS  FOR  INVENTIONS 


29 


granted  to  S.  T.  Fowler,  in  1860,  No.  28,069,  where  the  concrete 
wall  is  to  be  strengthened  with  vertical  and  horizontal  timbers, 
to  be  buried  in  the  concrete;  a  more  rational  construction  is 


FIGURE  38. 


proposed  in  1862,  No.  37,134,  by  G.  H.  Johnson,  for  grain-bins: 
"  a  new  construction  formed  of  brick- work  tied  together  by 
plates  and  rods  of  iron."  In  1869,  No.  87,569,  G.  H.  Johnson 


FIGURE 


improved  this  construction,  using  "  horizontal  annular  tension- 
bars  .  .  .  the  ends  of  each  bar  being  so  united  as  that  it  shall 
form  an  endless,  unbroken  band  ...  in  the  combination  .  .  . 
with  .  .  .  vertical  connecting-rods  so  as  to  form  a  metallic 


30 


REINFORCED   CONCRETE  BUILDINGS 


frame  within  the  walls  of  the  structure."  This  invention  (Fig- 
ure 39)  was  not  the  only  important  improvement  of  that  day; 
in  1868,  C.  Williams,  No.  75,098,  invented  the  metal  lattice- 
reinforcement  for  concrete  walls.  The  lattice-work  was  built 
up  by  riveting  the  slats  together  (Figure  40). 

The  first  use  of  concrete  in  columns  must  be  conceded  to 
W.  H.  Wood  who,  in  1862,  No.  36,747,  patented  an  improvement 
in  piers  and  bridges.  The  invention  consists  in  the  use  of  hol- 
low cast-iron  columns  filled  with  concrete  or  cement,  and  sup- 
ported on  wooden  spiles  below  the  surface  of  the  bed  of  the 
river.  The  first  ceiling  was  proposed  by  J.  Gilbert,  1867,  No. 
64,659.  This  patent  shows  corrugated  iron  plates  filled  with 


FIGURE  40. 

concrete,  the  concrete  to  extend  an  inch  or  so  above  the  top  of 
the  corrugation  (Figure  41).  His  solution  of  the  problem 
" self-centering  reinforcement"  is  not  very  inferior  to  those 
proposed  by  more  recent  inventors.  Thus  we  see  that  around 
the  year  1870  the  combination  of  masonry  of  various  kinds 
with  a  strengthening  metal  work  was  quite  well  known.  The 
patent,  No.  88,547,  granted  to  F.  Coignet,  a  Frenchman,  in 
1869,  states  the  general  principles  very  clearly:  "  In  the  body  of 
artificial  stones":  "skeletons  or  metallic  framework,  linked  or 
arranged  so  as  to  strengthen  the  same."  This  is  the  whole 
science  of  reinforced  concrete  construction  in  few  words.  As 
an  example,  he  proposes  to  use  a  cylindrical  web  of  small  rod- 
iron  or  wire  in  combination  with  a  cement  envelope,  for  the  pur- 
pose of  resisting  the  interior  pressure  in  pipes,  as  well  as  T-  or 


BASIC  PATENTS  FOR  INVENTIONS 


31 


L-irons  for  other  purposes.  The  series  of  patents  granted  to 
Coignet  in  1869  deserve  more  than  usual  attention,  as  they 
contain  much  good  advice  of  value  to  engineers;  they  are 
No.  88,545,  88,546,  88,547,  88,548,  and  88,549. 


FIGURE  41. 

The  brick  arch  with  abutment-shoe  and  tension  bar  between 
abutments  was  invented  by  C.  Henderson,  in  1871,  No.  113,881 
(Figure  42);  the  brick  arch  reinforced  on  the  cantilever  prin- 
ciple was  invented  by  F.  Alsip,  No.  120,608,  in  the  same  year. 
It  is  not  clear  from  the  description  whether  Alsip  really  con- 


FIGURE  42. 

sidered  his  invention  as  a  cantilever  construction,  but  the  fact 
remains  that  all  the  essential  elements  of  a  cantilever  are  pres- 
ent in  this  patent.  A  very  interesting  patent  No.  122,498  is  the 
one  granted  to  W.  H.  Smith,  in  1872,  for  a  concrete  pavement. 
On  soft  ground,  the  arched  pavement  is  intended  to  be  self- 
supporting.  Tie-rods  are  then  carried  under  the  pavement  from 


curb  to  curb,  or  "chords  may  be  embedded  in  the  composition 
to  operate  in  lieu  of  abutments  to  the  arch."  In  the  drawing, 
the  tension  rod  is  shown  provided  with  a  large  button  on  the 
end,  evidently  for  the  purpose  of  preventing  slipping  of  the 
bar  (Figure  43). 


32 


REINFORCED  CONCRETE  BUILDINGS 


The  patent  issued  to  Sisson  and  Wetmore,  in  1872,  No. 
124,453  (Figure  44),  shows  "a  combination  of  trussed  and  un- 
trussed  frames  of  light  bar-iron  to  form  skeleton  wall-posts, 
girders,  etc.,  in  combination  with  a  filling  of  beton  or  other 
suitable  concrete,  to  be  poured  in  a  state  more  or  less  liquid. 
Our  object  is  to  have  the  beton  and  iron  frames  furnish  mutual 
support  and  protection  to  each  other."  Considered  as  a  beam, 


FIGURE  44. 

the  wall-post  of  this  patent  exhibits  many  of  the  essential  fea- 
tures of  present  day  practice:  the  top  bar  extending  from  one 
span  to  another,  the  trussed  bar  bent  up  over  the  support,  the 
horizontal  lacing  of  the  verticals  and  the  vertical  lacing  of  the 
horizontals,  etc. 

But  generally  speaking,  the  reinforced  brick-arch  continues 
to  hold  the  interest  of  the  inventors.  In  1872,  P.  H.  Jackson 
received  a  patent,  No.  126,396  (Figure  45),  for  a  peculiar  con- 
struction of  abutment-casting  to  be  used  in  connection  with 
reinforced  arches,  and  in  1873,  No.  137,345,  N.  Cheney  proposes 


BASIC  PATENTS  FOR  INVENTIONS 


33 


to  make  the  tension  reinforcement  of  light  wires  placed  close 
together  and  interwoven  with  cross-wires,  to  serve  the  addi- 
tional purpose  of  a  metallic  lathing.  The  earthquake-proof 
house  invented  by  D.  L.  Emerson,  in  1873,  No.  137,833,  calls 


FIGURE  45. 

for  vertical  rods  or  plates  in  the  walls,  and  anchors  passing 
through  them,  the  plates  and  anchors  being  connected  with 
strap  iron.  In  the  same,  year  J.  W.  Basset,  No.  138,118  (Fig- 
ure 46)  shows  a  construction  of  individual  plaster  slabs  with  a 
metallic  trellis  work  within,  the  ends  of  which  extend  beyond 
the  block,  for  the  purpose  of  locking  the  various  blocks  together. 
While  not  strictly  within  the  scope  of  this  paper,  attention 
is  called  to  the  patent,  No.  172,641,  granted  to  O.  C.  Matthews, 


n 


FIGURE  46. 

in  1876,  for  a  foundation,  in  which  piles  are  driven  and  again 
withdrawn  and  the  holes  filled  with  concrete  (Figure  47). 

In  1878,  T.  Hyatt,  No.  206,112,  ended  the  "  period  of  dis- 
covery "  and  put  the  theory  of  reinforced  concrete  construction 
on  a  rational  basis,  and  at  the  same  time  received  a  patent  of 
remarkably  broad  scope,  covering  practically  the  entire  field 
of  reinforced  concrete  and  masonry  construction.  The  general 
purport  of  this  invention  is  set  forth  in  a  volume  entitled  "An 
account  of  some  experiments  with  Portland  Cement  concrete, 
combined  with  iron,"  of  which  a  copy  was  deposited  in  the 
library  of  the  Patent  Office,  but  which  was  otherwise  designed 
for  private  circulation.  Hyatt  appears  to  be  the  first  to  state 
specifically  that  the  steel  must  be  able  to  resist  sufficient  tensile 


34 


REINFORCED  CONCRETE  BUILDINGS 


stress  to  balance  the  compressive  stresses  on  the  concrete,  that 
all  metal  may  be  dispensed  with  save  the  tension  rod  only,  that 
both  baked  bricks  and  concrete  possess  in  themselves  cohesive 
power  and  strength  sufficient  to  perform  the  functions  ordina- 
rily performed  by  the  metallic  web.  He  realizes  the  value  of 
deformed  bars  and  says:  "  I  prefer  to  use  metal  specially  rolled 
for  the  purposes,  with  bosses  or  raised  portions  formed  upon 


FIGURE  47. 


FIGURE  48. 


the  flat  faces  of  the  metal.  When  I  make  use  of  common  bar 
or  hoop  iron,  I  stud  the  slips  with  pins;  or  I  make  use  of  several 
blades  threaded  upon  wires,  as  represented  by  Figure  1."  In 
the  book  mentioned  above,  he  laid  down  the  results  of  his  ex- 
periments which  led  him  to  bend  some  of  the  bars  up,  and  also  to 
use  a  rigidly  attached  separate  "  shear  member."  The  analysis 
is  very  complete,  both  in  his  book  and  in  his  patent  speci- 
fication. He  reinforces  his  columns  with  longitudinals  or  hor- 
izontal hoops,  as  the  case  may  require,  or  both.  He  says:  "In 


BASIC  PATENTS  FOR  INVENTIONS 


35 


constructing  the  columns  or  piers  wholly  of  concrete,  I  make  the 
structure  solid,  the  concrete  then  bearing  the  load,  and,  giving 
way  under  compression,  would  naturally  incline  to  yield  in  the 
first  place,  not  from  absolute  crush  of  the  materials,  but  from 
want  of  sufficient  tensile  resistance  at  the  circumference  of  the 
column.  But  this  tendency  being  resisted  by  the  circular 
ties,  such  a  concrete  could  give  way  only  by  the  crush  of  its 
particles."  In  short,  the  whole  theory  of  hooped  columns. 
The  only  difference  is  that  Considere  prefers  the  use  of  spirally 
wound  reinforcement,  while  Hyatt  uses  the  individual  bands 
(Figure  48). 

To  what  extent  Hyatt  was  familiar  with  Pasley's  tests,  if 
at  all,  we  do  not  know;  in  his  book  of  1877  he  gives  a  brief  ac- 
count of  the  history  of  fireproof  construction,  but  gives  no  ref- 
erence whatever  to  the  tests  just  mentioned.  It  appears  that 
he  had  a  test  made  in  September,  1855,  in  New  York,  under 
the  general  supervision  of  Mr.  R.  G.  Hatfield;  the  beam  was 
about  9"  square,  and  had  a  tie-rod  passing  through  holes  made 
for  the  purpose  in  the  bottoms  of  the  bricks.  More  important 
tests  were  made  by  Kirkaldy  in  London,  from  1874  to  1877,  on 
beams  made  by  Hyatt. 

The  Period  of  Improvement.  Broadly  speaking,  the  Hyatt 
Patent,  No.  206,112,  shows  and  describes  everything  necessary 


FIGURE  49. 

for  the  practical  use  of  reinforced  concrete,  and  the  patents 
of  the  following  period  are  therefore  mainly  for  improvements, 
many  of  which  are  due  to  Hyatt.  Most  interesting  is  the  one 
granted  in  1883,  No.  290,886,  for  a  concrete  floor,  showing  not 
only  transverse  arches  between  the  ribs,  but  also  the  use  of  web 
reinforcement  in  a  continuous  sheet  along  the  center  of  the 
beam.  In  1881  a  patent,  No.  237,471,  was  granted  to  S.  Bis- 
sell  (Figure  49)  for  an  arch-bridge,  showing  diagonal  straight 
reinforcement  within  the  masonry,  the  object  being  to  construct 


36  REINFORCED  CONCRETE  BUILDINGS 

"an  arch  of  limited  span  without  causing  any  horizontal  thrust 
upon'  the  abutments."  The  Cubbins  patent  of  1883,  No. 
285,801,  shows  a  circular  cistern  cover  "of  artificial  stone, 
having  a  metallic  band  or  tire"  (Figure  50)  or  "consisting  of 


FIGURE  50. 

a  concavo-convex  or  arched  disk  .  .  .  inclosed  by  a  metallic 
band  or  tire."  This  appears  to  be  the  first  slab  with  "  cir- 
cular reinforcement."  "Expanded  metal"  was  patented, 
No.  297,382,  in  1884,  by  J.  F.  Golding:  "metallic  screening 
formed  of  slashed  and  stretched  metal."  The  particular  use 
to  which  the  invention  was  to  be  put  is  not  specified,  and  at 
first  it  was  used  exclusively  as  a  metal  lath.  Its  use  as 
reinforcement  for  structural  concrete  is  of  much  later  date 
(Figure  51). 


FIGURE  51. 

A  number  of  interesting  patents  are  granted  at  various 
dates  to  P.  H.  Jackson.  The  first,  No.  302,338,  in  1884,  is  not 
of  interest  in  this  connection;  it  shows  principally  the  usual 
tie-rod  construction  in  a  brick  arch.  But  the  following  year, 
1885,  he  took  out  a  patent,  No.  314,677,  showing,  for  the  first 
time,  the  bent-up  or  "trussed"  arrangement  of  reinforcement 
(Figure  52);  the  bars  are  carried  to  the  support  where  they  are 
anchored  by  means  of  nuts.  The  concrete  and  its  reinforcement 
rest  upon  corrugated  iron  plates,  and  the  bars  may  be  secured 


BASIC  PATENTS  FOR  INVENTIONS 


37 


or  not  at  intervals  to  the  bottom  of  the  corrugated  plates. 
Another  patent,  No.  320,066,  of  the  same  year,  shows  the  rein- 
forcement continued  into  the  adjacent  bay  and  there  hooked 


FIGURE  52. 

over  the  tops  of  the  I-beams  (Figure  53),  which  here  have  the 
function  of  the  main  girders.  The  patent,  No.  339,296,  of  1886, 
specifies  an  expansion  joint  in  the  construction  of  a  reinforced 
concrete  arch;  evidently  the  troubles  caused  by  expansion  and 
shrinkage  were  well  known  at  this  early  date.  Two  patents, 
Nos.  366,839  and  366,840,  were  taken  out  in  1887  for  "  series 


FIGURE  53. 

of  arches  composed  of  concrete,  and  a  longitudinal  tie  on  which 
the  footings  of  the  said  arches  are  supported  and  to  which  they 
are  fastened,"  and  a  construction  of  arches  with  longitudinal 
reinforcement  near  the  bottom;  these  arches  rest  on  one  side 
on  the  front  girder  of  the  building,  on  the  other  side  upon  the 
area  wall.  Also  from  that  year  date  the  following  patents: 
two,  Nos.  367,343  and  370,625,  showing  the  application  of  dove- 
tailed corrugated  plates  filled  with  concrete  (Figure  54),  and 


FIGURE  54. 


three,  Nos.  371,843,  371,844,  and  ?71,845,  showing  the  use  of 
I-beam  reinforcement  in  the  bottom  of  the  beam,  as  well  as 


38 


REINFORCED  CONCRETE  BUILDINGS 


compression-reinforcement  in  the  top  (Figure  55).  The  reis- 
sued, RIO, 921,  and  the  original  patent,  No.  375,999,  issued  in 
1888,  may  be  noted  in  passing. 

When  we  consider  the  state  of  the  art  as  it  appears  from 
the  patents  mentioned  above,  the  Monier  patent  of  1884,  No. 
302,664  (Figure  56),  cannot  be  called  much  of  an  improvement. 


FIGURE  55. 


FIGURE  56. 


Nevertheless,  the  name  Monier  was  for  many  years  synonymous 
with  "  reinforced  concrete,"  at  least  in  Europe,  where  the  Mon- 
ier patents  were  bought  and  greatly  developed  by  German 
engineers.  "My  invention,"  he  says,  "relates  to  the  use  and 
sale  of  integral  elements  of  construction  of  metal  and  concrete 
or  mortar  combined,  the  mortar  forming  the  covering  for  a 
metal  skeleton.  This  skeleton  is  composed  of  longitudinal 
bars  or  rods  and  transverse  ribs,  secured  together  by  metal 
ligatures."  The  Monier  patent,  No.  486,535,  of  1892,  is  practi- 


FlGURE    57. 

cally  nothing  but  a  series  of  special  designs  based  upon  this  same 
principle,  and  contains  little  new  material.  Yet  a  great  industry 
was  based  both  here  and  in  Europe  upon  the  Monier  patents. 

The  Ransome  patents  have  been  described  in  an  earlier 
chapter  and  are  not  referred  to  here. 

The  "trussed"  arrangement  of  the  steel  was,  as  stated 
above,  invented  by  Jackson  in  1885.  The  Gustavino  patent, 
No.  336,048,  of  1886  (Figure  57)  shows  the  same  feature,  as 


BASIC  PATENTS  FOR  INVENTIONS 


39 


well  as  the  rod  with  a  continuous  curve  between  supports.  In 
addition  to  this  tie-rod  which  extends  from  wall  to  wall,  "  I 
may  in  practice  use  a  straight  tie-rod  extending  between  wall 
and  wall  above  the  arch."  The  same  year,  1886,  saw  the  origin 
of  another  new  type  of  construction  which  stands  on  the  border 
between  reinforced  concrete  and  plaster  work.  The  Rabitz 
construction,  No.  339,211  (Figure  58),  calls  for  a  metallic  skele- 


FIGURE  58. 

ton  frame  of  vertical  rods  and  a  reticulated  metallic  netting, 
in  combination  with  a  suitable  coating  of  cement  mortar  or  sim- 
ilar material.  In  the  patent  issued  to  P.  M.  Bruner,  No.  356,703, 
in  1887,  something  approaching  the  U-bar  (Figure  59)  is  shown; 


FIGURE  59. 

although  the  construction  would  not  be  classed  as  reinforced 
concrete  at  this  present  time,  the  rods  being  disposed  princi- 
pally on  the  compression  side,  from  which  rods  transverse  ties 
hang  down  in  the  beam.  A  telegraph  pole  was  invented  by 
D.  Wilson,  No.  374,103,  in  1887;  it  was  to  be  composed  of  a  skel- 
eton frame  having  rods  and  horizontal  hoops,  and  a  coating  or 
body  of  cement  inclosing  the  frame.  The  same  idea  was  pat- 
ented, No.  411,360,  in  1889,  by  O.  A.  Stempel,  who  claims  a 
post,  rail-tie,  or  beam,  composed  of  "a  metal  frame,  the  filling 
and  inclosure  of  imperishable  material  that  protects  said  frame 


40  REINFORCED  CONCRETE  BUILDINGS 

from  the  inroads  of  moisture  and  rust,  and  said  frame  arranged 
to  protect  said  structure  from  breakage."  The  drawing  looks 
somewhat  like  what  an  engineer  would  prepare  for  a  column  at 
this  time  (Figure  60). 

The  patents  granted  to  M.  F.  McCarthy  show 
again  "the  combination  (with  an  I-beam  supporting 
the  slab)  of  the  wire  strands  extending  over  and 
drooped  between  the  same,  and  the  concrete  filling 
wherein  said  beams  and  strands  are  embedded."  This 
quotation  is  from  the  patent  issued  in  1891,  No. 
455,687  (Figure  61);  the  four  patents,  Nos.  520,489, 
520,490,  520,491,  and  520,492,  issued  in  1894,  show 
various  combinations  and  variations  of  the  same  prin- 
ciple. The  patent  issued  to  P.  Cottancin,  No.  459,944, 
in  1891,  is  for  a  strengthening  web  "  characterized 
by  the  union  in  a  reticulated  fabric  of  a  warp  and  a 
weft,  each  composed  of  a  wire,  band,  or  bar  bent  on 
itself  into  a  sinuous  or  like  shape."  This  patent 
forms  the  base  for  a  large  industry  especially  in 
France.  The  J.  Melan  patent,  No.  505,054,  of  1893 
(Figure  62),  claims  "a  vault  or  arch  consisting  of 
abutments,  beams,  or  girders,  arched  ribs  rigidly  con- 
nected with  said  abutments,  beams,  or  girders,  and 
a  filling  of  concrete  or  the  like  between  said  ribs." 
A  number  of  arch-bridges  have  been  constructed 
under  this  patent.  A.  L.  Johnson  patented,  No. 
550,177  (1895),  a  construction  of  floors  much  used 
at  one  time  in  the  West,  comprising  mainly  I-beams 
with  suspension  straps  fastened  at  the  tops  of  the 
beams  and  drooping  between  the  beams;  the  straps 
are  flat  and  support  the  concrete  rib  of  the  beam 
(Figure  63),  upon  which  in  turn  rests  the  concrete 
slab.  Another  important  arch-patent,  No.  583,464, 
was  granted  to  F.  von  Emperger,  in  1897,  for  an  im- 
provement in  the  Melan  patent  described  above;  it 


FIGURE  60.  consists  mainly  in  using  two  ribs  instead  of  one,  each 
rib   being  placed  near  one   surface  of  the  concrete. 
Secondary  members   connect   the   top   and  bottom    ribs   (Fig- 
ure 64). 

Recent  Patents.     The   idea  of  molding  reinforced  concrete 


BASIC  PATENTS  FOR  INVENTIONS 


41 


members    separately    and    afterwards    erecting   them    in   place 
appears  to  be  almost  as  old  as  the  art  itself,  and  a  number  of 


FIGURE  61. 


the  patents  mentioned  above  refer  to  this  possibility  without 
going  much  into  the  details.     In  1898,  a  patent,  No.  606,696, 


FIGURE  62. 

was  issued  to  G.  B.  Waite  for  a  beam  construction  (Figure  65), 
the  sole  object  of  which  is  to  provide  members  adapted  to  be 


FIGURE  63. 

molded  in  advance  and  erected  in  place  after  hardening.     The 
individual  sections  are  made  of  I-shape  and  reinforced  in  top 


FIGURE  64. 

and  bottom,  or  in  the  bottom  only;   " shear"  members  of  vari- 
ous forms  are  used  in  the  beam-webs.     The  De  Man  twisted 


FIGURE  65. 
bar  was  patented,  No.  606,988,  in  the  same  year;  it  consists 


42 


REINFORCED  CONCRETE  BUILDINGS 


of  "a  thin  flat  bar  having  twists  formed  therein  at  intervals" 
(Figure  66). 

The  patents  granted  to  F.  Hennebique,  in  1898,  are  three  in 
number.     The  first,  No.  611,907,  shows  the  now  almost  univer- 


FIGURE  66. 


sally  used  combination  of  open,  U-formed  shear  members  with 
horizontal  and  trussed  main  reinforcement,  with  the  main  bars 
extending  into  the  adjacent  span  (Figure  67).  While  the 


FIGURE  67. 

authorities  seem  to  disagree  in  regard  to  the  value  of  the  pro- 
tection afforded  by  this  patent,  there  is  not  the  slightest  reason 
to  doubt  that  this  construction  has  been  of  the  greatest  benefit 
to  the  art.  The  second  Hennebique  patent,  No.  611,908,  is 
for  a  system  of  separately  molded  members,  claiming  in  sub- 
stance a  combination  of  joists  and  "  a  plurality  of  slabs  having 
projecting  cores  embedded  in  said  joists  "  (Figure  68);  the  word 


FIGURE  68. 

core  means  here  the  reinforcing  bar,  and  the  slabs  are  placed 
with  their  ends  resting  upon  the  side-forms  for  the  joists,  so 


BASIC  PATENTS  FOR  INVENTIONS 


43 


that,  when  concrete  is  poured  in  the  joist-molds,  the  project- 
ing ends  are  embedded  in  the  concrete.  The  third  patent, 
No.  611,909,  is  for  a  pile  of  reinforced  concrete  having  grooves  in 
two  faces,  so  that  a  tight  cofferdam  may  be  made  by  using  the 
piles  for  sheet  piling,  and  filling  in  the  grooves  with  grout.  The 
structures  erected  under  the  Hennebique  patents  are  numbered 
by  the  hundreds  in  any  one  of  the  several  civilized  countries. 

The  patent,  No.  617,615,  issued  to  E.  Thacher,  in  1899  (Fig- 
ure 69),  for  an  arch  construction,  claims  the  combination  of  the 


FIGURE  69. 

concrete  arch  with  its  abutments,  and  reinforcing  bars  in  pairs, 
one  bar  near  the  intrados,  and  one  near  the  extrados,  the  two 
bars  of  each  pair  to  be  above  one  another,  either  both  or  only 
one  of  these  bars  to  extend  well  into  the  abutment,  and,  in  par- 
ticular, "each  bar  of  a  pair  to  be  independent  of  the  other." 
A  comparison  with  the  Melan  and  v.  Emperger  patents  is  of 
interest,  as  the  bars  in  the  v.  Emperger  patent  extend  into  the 
abutments  and  are  placed  one  above  the  other.  In  the  same 
year,  1899,  a  patent  (Figure  70),  No.  634,986,  was  granted  to 


FIGURE  70. 

A.  Matrai  for  a  system  of  wire  reinforcement  embodying  many 
interesting  features.  One  object  of  the  construction  is  to  unload 
as  far  as  possible  the  middle  of  the  supporting  beam  or  girder, 
and  these  again  are  reinforced  with  a  number  of  suspension 
cables  or  wires.  This  construction  is  in  considerable  favor  in 
Europe.  In  1900,  a  patent,  No.  654,683,  was  issued  to  I.  A. 
Shaler,  for  a  construction  embodying  the  use  of  longitudinal  and 
transverse  rods,  the  latter  welded  to  the  main  bars  at  intervals, 


44 


REINFORCED  CONCRETE  BUILDINGS 


and  in  the  same  year,  L.  G.  Hallberg  had  a  patent,  No.  659,967, 
issued  for  a  foundation  built  on  the  principle  of  "  circular  rein- 
forcement "  (Figure  71)  in  combination  with  radial  bars.  The 


Wayss  patent,  No.  673,310  -  72)  of  1901,  is  of  interest,  on 

account  of  the  rigidly  attach  ear  members  and  other  fea- 

tures, the  purpose  being  to  ^otain  similar  advantages  as  out- 
lined for  the  Hennebique  patent  without  infringing  the  same; 


FIGURE  72. 

the  construction  is  dissimilar  to  Hennebique  in  the  particular 
arrangement  of  the  parts.  The  well-known  Thacher  bar  was 
patented  in  1902,  No.  691,416  (Figure  73),  and  in  the  same  year 


BASIC  PATENTS  FOR  INVENTIONS 


45 


a  patent,  No.  709,794  (Figure  74),  was  granted  to  W.  C.  Farm- 
ley  for  a  concrete  arch  construction,  in  which  the  steel  is  so 
arranged  as  to  make  the  same  bar  pass  from  the  tension  region 


FIGURE  73. 

near  the  intrados  to  the  tension  region  near  the  extrados,  etc. 
The  Visintini  patent,  No.  735,920,  of  1903,  shows  the  peculiar 
type  of  construction  known  under  that  name;  instead  of  the 


FIGURE  74. 

ordinary  solid  beam,  a  lattice-girder  of  reinforced  concrete  is 
used.  The  top  and  bottom  flanges  are  reinforced  with  longi- 
tudinal bars,  and  the  cross-bars  are  embedded  in  the  concrete 


FIGURE  75. 


work  of  the  lattices  (Figure  75).  The  Visintini  beam  has 
been  used  but  little  in  this  country,  but  abroad  a  large  number 
of  structures  have  been  erected  under  this  patent.  In  1903, 


FIGURE  76. 

the  first  Kahn  patent,  No.  736,602,  was  issued,  to  be  followed 
by  many  more  (Figure  76).     The  principal  features  are  well 


46 


REINFORCED  CONCRETE  BUILDINGS 


known:  The  rigidly  attached  secondary  members  are  manu- 
factured in  one  piece  with  the  main  tension  rod,  then  sheared 
loose  from  the  main  body  along  the  greater  part  of  the  length 


FIGURE  77. 

of  the  rod  and  bent  up  as  desired.  The  Weber  chimney-con- 
struction was  patented  in  1903,  No.  748,242;  the  lower  portion 
of  the  chimney  is  provided  with  a  circumferential  air-space 


open  at  its  base  to  the  outer  air  and  leading  at  its  upper  end  into 

the  chimney  flue  at  the  base  of  the  upper  single  flue  (Figure  77). 

A.  Considere  took  out  a  patent,  No.  752,523,  for  his  well- 


BASIC  PATENTS  FOR  INVENTIONS  47 

known  column  construction,  claiming  "a  solid  concrete  core 
with  independent  helicoidal  coils  of  metal  surrounding  said 
core,  and  arranged  very  close  together,"  and  also  the  combina- 
tion of  these  elements  with  separate  longitudinal  rods,  in  1904 
(Figure  78).  With  this  patent  we  may  consider  the  period  of 
invention  as  coming  to  an  end.  A  very  large  number  of  patents 
have  been  granted  since,  mostly  for  slight  improvements,  and 
an  enumeration  of  all  these  details  would  be  very  tedious  and 
without  serious  importance,  although  several  patents  of  the 
greatest  interest  may  be  found  in  this  great  mass  of  dead 
material. 


PART   II 

RATIONAL  DESIGN  OF  REINFORCED 
CONCRETE  BUILDINGS 

BY  ALEXIS  SAURBREY 


CHAPTER   III 
INTRODUCTION 

1.  EXPERIENCE  teaches  that  concrete  beams  may  be  greatly 
strengthened  by  introducing  a  comparatively  small  amount  of 
steel  within  the  concrete,  according  to  certain  principles  of  which 
the  following  is  a  discussion.     This  combination  of  concrete  and 
steel  is  called  Reinforced  Concrete;  the   essential  peculiarity  of 
reinforced  concrete  structures  is  that  both  the  concrete  and  the 
steel,  if  alone,  would  be  grossly  inadequate  for  the  load  which 
they  will  carry  when  combined;  the  load  carrying  capacity  is 
not  the  sum  of  the  individual  capacities  of  the  concrete  and  the 
steel.     This  general  rule  is  not  without  exception,  if  structures 
like    the    ordinary    reinforced    concrete    column    are    included; 
strictly  speaking,  only  the  hooped  column  is  entitled  to  be  clas- 
sified as  reinforced  concrete,  because  in  that  case  a  small  amount 
of  steel  added  to  the  concrete  changes  the  structural  properties 
of  the  column  entirely. 

2.  The  stresses  in  a  reinforced  concrete  structure  are  neces- 
sarily  complicated.     Not  only  is  the  steel  entirely  dissimilar 
in  nature  to  the  concrete  which  it  reinforces,  but  the  concrete 
itself  is  not  homogeneous  in  the  strictest  sense  of  the  word. 
Yet  two  cubes  of  large  size,  cut  from  different  parts  of  the  beam, 
must  be  assumed  to  be  theoretically  alike;  —  we  make  there- 
fore the  necessary  and  justified  assumption  that  the  lack  of  homo- 
geneity of  the  concrete  is  of  second  order  as  compared  with  that 
of  the  structure  as  a  totality:  necessary,  because  otherwise  we 
cannot  advance  any  theory;   justified,   because  the  differences 
between  the  nature  of  steel  and  concrete  are  sufficiently  large 
to    overshadow    completely    the    small    differences    which    un- 
doubtedly exist  within  the  concrete  itself. 

3.  Generally,  the  properties  of  reinforced  concrete  are  known 
when  the  properties  of  the  two  materials  are  known;  —  there  is 

51 


52  REINFORCED  CONCRETE  BUILDINGS 

no  reason  for  believing  that  the  properties  of  either  material 
are  changed  in  any  way  by  the  presence  of  the  other.  It  is, 
however,  necessary  to  expand  the  limits  of  our  research  when 
dealing  with  a  combination  of  two  materials,  because  the  prop- 
erties of  the  combination  depend  primarily  upon  the  ability  of 
the  two  materials  to  co-operate,  and  only  in  second  line  upon 
their  individual  properties  such  as  strength,  elasticity,  etc. 
This  co-operative  ability  is  of  a  somewhat  obscure  nature;  — • 
without  making  any  attempt  of  explaining  it,  we  must  admit 
its  existence.  In  the  following  it  is  referred  to  as  the  "bond" 
or  the  "adhesion."  When  this  bond  is  broken  the  structure 
fails. 

4.  The  purpose  of  design  is  to  produce  not  only  a  structure 
of  adequate  strength,  but  one  of  equal  strength  in  its  several 
parts.     With  consistent  formulas  for  the  various  elements,  the 
allowable  stresses  should  therefore  be  the  same  for  all  elements 
of  the  structure  considered.     Experience  shows,  however,  that 
the  difficulties  to  be  overcome  in  the  erection  are  different  for 
different  parts;  we  can  readily  see  that  a  local  deposit  of  bad 
concrete  as  large  as  a  hand  will  affect  a  10"  column  and  a  6" 
floor  slab  in  dissimilar  ways.     This  is  the  reason  for  variable 
allowable  stresses  —  in  any  case  the  purpose  of  fixing  certain 
maximum  stresses  is  to  insure  an  ample  factor  of  safety.     For- 
tunately the  investigation  of  stresses  in  a  given  beam  is  very 
much  simpler  under  moderate  loads  than  near  ultimate  failure; 
the  coefficient  of  elasticity  for  steel  Ea  is  a  constant,  and  that 
for   concrete   Ec  varies    but   slightly.     For  practical   purposes 
the  ratio  ES/EC  —  r  is  assumed  to  be  a  constant  up  to  the  limit 
of  the  allowable  stresses.     Within  this  same  limit  we  assume 
sections  plane  before  the  load  was  put  on  to  remain  plane  under 
load,  and  we  assume  proportionality  between  stress  and  deform- 
ation.    The  tensile  strength  of  the  concrete  is  entirely  disre- 
garded.    None  of  these  assumptions  can  be  called  absolutely 
correct;  they  are,  however,  no  more  inaccurate  than  any  other 
set  of  assumptions  which  we  would  be  able  to  suggest  in  our 
present  state  of  knowledge;  moreover,  they  are  the  simplest 
possible. 

5.  As  the  tensile  strength  of  concrete  is  much  less  than  its 
compressive  strength,  the  principle  is  to  utilize  the  available 
compressive  resistance  and  use  steel  bars  to  carry  the  tension. 


INTRODUCTION  53 

Sometimes  steel  is  also  used  in  compression,  although  with  less 
success,  the  object  being  to  limit  the  size  of  the  columns  and  to 
fortify  them  against  excentric  loads.  We  shall  see  later  that 
it  is  possible  to  construct  a  column  in  which  the  steel  is  stressed 
in  tension  (Article  12). 

6.  In  any  kind  of  concrete  structure  the  embedded  steel 
has  a  tendency  to  displacement  in  its  own  longitudinal  direc- 
tion under  load.  The  value  of  the  steel  as  reinforcement  de- 
pends upon  its  ability  to  withstand  any  forces  tending  to  either 
push  or  pull  it  out;  reinforced  concrete  is  an  impossibility 
without  adhesion  between  steel  and  concrete,  and  destruction 
of  the  bond  or  adhesion  means  failure.  The  law  governing 
adhesion  is  therefore  the  foundation  of  all  theoretical  study  of 
reinforced  concrete. 


CHAPTER   IV 

ADHESION 

7.  THE  adhesion  is  measured  in  Ibs.  per  square  inch  of  em- 
bedded surface  of  the  rod;  its  value  is  different  for  pulling  and 
pushing  tests.  As  the  latter  is  somewhat  higher  it  is  sufficient  to 
investigate  the  laws  governing  the  pulling  resistance  and  apply 
these  laws  to  the  pushing  resistance  also.  The  mathematical 
analysis  of  the  bond  stresses  is  impossible  with  the  material  on 
hand;  even  the  test-data  are  meager  and  often  contradictory. 
We  know,  however,  that  the  following  statements  are  approxi- 
mately true,  so  that  an  embedded  rod  pulls  out  of  the  concrete 
block : 

(a)  when  the  stress  in  the  steel  reaches  the  elastic  limit  of 
the  steel. 

(6)  when  the  tensile  resistance  of  the  concrete,  in  a  lateral 
direction,  is  reached,  because  the  block  splits. 

(c)  when,  instead  of  splitting,  the  concrete  around  the  rod 

expands  sufficiently  to  let  the  irregularities  of  the  rod 
pass  through. 

(d)  when  the  adhesion  is  destroyed. 

Obviously,  then,  the  designer  must  keep  the  steel  stress  well  below 
the  elastic  limit,  allowing  for  this  and  other  reasons  an  ample 
factor  of  safety,  while,  at  the  same  time,  the  concrete  must  be 
strong  enough  to  meet  the  demands  made  upon  it.  Hence  the 
diameter  of  the  concrete  block,  or  the  thickness  of  the  piece,  is  a 
very  important  factor,  but  unfortunately  nothing  is  known  in 
regard  to  the  minimum  allowable  diameter,  except  that  it  is 
greater  for  deformed  bars  than  for  plain  round  or  square  rods. 
We  can  readily  see  that  both  the  tensile  strength  and  the  coeffi- 
cient of  elasticity  of  the  concrete  has  great  influence  upon  the 
minimum  allowable  diameter;  with  a  well-proportioned  mortar 
and  a  mixture  of  say  1 :2:3J,  we  may  perhaps  suggest  a  diameter 
of  concrete  equal  to  ten  diameters  of  the  embedded  steel  as 

'  54 


ADHESION  55 

reasonably  safe.  In  floor  construction  the  bars  usually  find 
their  ultimate  anchorage  in  much  larger  bodies,  the  slab  bars 
passing  through  the  beams,  the  beam  bars  through  the  girders, 
and  finally  the  girder  bars  through  the  columns.  In  all  these 
cases  the  concrete  is  reinforced  in  a  direction  transverse  to  the 
direction  of  the  pull,  and  the  expansion  in  a  lateral  direction  is 
thus  partially  or  entirely  prevented. 

8.  In  the  beam  theory  to  be  outlined  below,  great  importance 
is  attached  to  the  length  of  embedment  beyond  the  supports  of  the 
beam,  in  fact,  this  length-  represents  the  ultimate  reserve  of 
strength  of  the  beam.  It  is  usually  considered  good  practice 
to  imbed  the  bent  bars  from  twenty-four  to  thirty-six  diameters 


i 

6  Diameters 

Y 

J,   Diam.. 

V     1 

FIGURE 

-f  — 

79. 

beyond  the  support,  using  the  lower  figure  for  deformed  bars 
stressed  to  16,000  Ibs./sq.  inch,  and  the  higher  figure  for  deformed 
bars  stressed  to  20,000  Ibs./  sq.  inch.  For  plain  bars,  an  addi- 
tional hook  is  made  on  the  end  of  the  bar,  equal  in  length  to  six 
diameters.  In  many  cases  the  length  of  embedment  here  recom- 
mended cannot  be  obtained  for  the  reason  that  there  is  no  adjacent 
concrete  into  which  the  bars  may  be  extended,  as,  for  instance, 
in  the  case  of  a  beam  finding  its  bearing  in  an  outside  brick  wall. 
The  bars  are  then  hooked,  and  the  length  of  embedment  calcu- 
lated from  the  center  of  the  seat  to  the  end  of  the  bar,  including 
the  curved  end  of  the  hook.  Square  hooks  must  be  avoided,  a 
gentle  curve  of,  say,  six  times  the  radius  of  the  bar  is  much  more 
effective,  and  the  more  so  the  greater  the  radius  of  the  bent 
(Figure  79). 

9.  The  diameter  referred  to  in  the  preceding  paragraph  is 
not  the  diameter  of  each  individual  rod  or  bar,  unless  the  rods  be 
spaced  so  far  apart  that  each  will  pull  out  individually,  leaving 
the  concrete  intact  between.  The  diameter  is  that  of  a  circle  or 
other  curved  line  in  which  all  the  rods  may  be  enclosed  if  laid 
closely  together.  It  follows  that  it  is  good  practice  to  spread  the 
rods  out  as  much  as  possible;  in  a  beam  this  is  easily  obtained  by 


56  REINFORCED  CONCRETE  BUILDINGS 

bending  some  of  the  bars  up  over  the  support,  as  is  also  done  for 
other  important  reasons.  It  is  a  common  but  inexcusable 
mistake  to  use  a  number  of  small  diameter  rods  bunched  together; 
—it  is  almost  impossible  to  concrete  such  beams  properly,  and 
the  fallacy  of  the  argument  leading  to  such  construction  should 
be  evident. 


CHAPTER  V 
COMPRESSION  AND  LATERAL  EXPANSION 

10.  WITH  few  exceptions,  materials  submitted  to  deformation 
in  one  direction  undergo  deformations  in  all  other  directions. 
If  the  principal  deformation  is  a  shortening,  the  lateral  deforma- 
tion is  a  swelling,  which  must  be  taken  as  evidence  of  certain 
interior  stresses  in  the  body  in  a  direction  normal  to  that  of  the 
principal  stress.     These  transverse  stresses  are  of  the  greatest 
importance  for  materials  like  reinforced  concrete,  because,  if  not 
restrained,  they  bring  about  the  premature  failure  of  the  con- 
crete, while,  if  restrained,  they  may  be  used  to  increase  the 
strength  of  the  structure.     Thus,   as  pointed  out  above,  the 
transverse  swelling  affects  the  bond  of  an  embedded  rod;    if 
restrained  (by  surrounding  the  bar  with  a  coil  of  large  diameter) , 
the  value  of  the  bond  may  be  increased  as  much  as  fifty  per  cent, 
or  more.     Even  a  loose  stirrup  circling  the  tension  rod  at  the 
bottom  of  a  beam  increases  the  sliding  resistance  of  the  rod,  so  that 
a  rod,  covered  at  the  most  with  two  inches  of  concrete,  may  have 
the  same  sliding  resistance  as  one  embedded  in  a  large  body  of 
concrete.     Similarly,  the  Ransome  Coil  Coupling  may  be  used 
with  good  results  when  splicing  rods,  although  the  rods  should 
always  be  made  in  one  continuous  piece  whenever  it  is  practically 
possible.     The  coupling  is  made  simply  of  a  coiled  piece  of  very 
heavy  wire  or  a  light  bar  surrounding  the  splice  for  its  entire 
length,  which  should  be  equal  to  at  least  fifty  diameters  of  the 
rods  to  be  spliced  (Figure  14). 

11.  In  figure  80  a  short  block  is  shown  loaded  and  compressed 
in  one  direction,  thereby  shortening  the  length  of  the  vertical 
side  from  aa  to  bb.     We  notice  now  that  the  block  expands  in  a 
horizontal  direction,   the  diameter    increasing    from  cc  to  dd. 
It  requires  very  careful  observation  to  discover  this  swelling  in  a 
concrete  block,  which  usually  fails  along  a  diagonal  such  as  ae, 
but,  in  any  case,  experiments  with  greased  surfaces  have  shown 

57 


58  REINFORCED  CONCRETE  BUILDINGS 

that  when  the  friction  is  eliminated,  the  block  fails  along  vertical 
planes  such  as  ff.1  It  is  therefore  clear  that  longitudinal  rein- 
forcement in  the  direction  of  the  compressive  force  is  not  very 
efficient,  because  the  longitudinal  rods  simply  add  their  own 

1 

,b 


strength  to  that  of  the  concrete.  The  rods  act  as  slender  columns 
and  have  a  tendency  to  buckle,  so  that  if  no  other  provisions  are 
made,  the  strength  of  the  rods  is  practically  nil.  To  prevent 
buckling,  horizontal  ties  or  "  hoops  "  are  introduced,  but  it  is 
evident  that  unless  closely  spaced  the  hoops  are  of  little  value. 
If  therefore  the  column  or  block  is  to  have  vertical  reinforce- 
ment, it  must  have  closely  spaced  horizontal  hoops,  and  these  in 
turn  prevent  the  concrete  from  breaking  apart  along  the  vertical 
planes  ff  described.  In  this  way  the  hoops  become  a  very 
efficient  means  of  reinforcing. 

12.  In  order  to  understand  this  fully,  let  us  consider  a  cylinder 
filled  with  water,  one  end  being  equipped  with  a  water-tight  but 
frictionless  piston.  This  piston  will  carry  an  immense  weight 
on  its  upper  surface ;  in  fact,  the  entire  system  cannot  fail  before 
the  water  pressure  within  the  cylinder  exceeds  the  capacity  of 
the  cylinder  walls,  so  that  the  cylinder  bursts.  The  pressure 
within  the  cylinder  is  the  same  in  all  directions  per  unit  of  area; 
more  particularly  there  is  a  horizontal  (lateral)  pressure  on  each 
and  every  square  inch  equal  to  the  vertical  pressure  produced  by 
the  load  on  the  piston.  If  now  the  cylinder  is  filled  with  sand  in- 
stead of  water,  the  conditions  are  only  changed  to  this  extent 
that  the  lateral  pressure  against  the  walls  is  less  than  before,  so 
that  it  takes  a  greater  load  on  the  piston  to  burst  the  walls. 
Finally,  if  the  cylinder  is  filled  with  liquid  concrete,  and  the  con- 
crete is  allowed  to  set  hard,  the  pressure  on  the  walls  will  be  even 

1  According  to  tests  by  Foeppel  and  Mesnager.     See,  for  instance,  Con- 
sidere,  "Reinforced  Concrete,"  page  120. 


COMPRESSION  AND  LATERAL  EXPANSION  59 

less  than  before,  but  the  concrete  will  stand  much  higher  pressure 
when  enclosed  in  the  cylinder  than  when  free.  This,  then,  is  the 
principle  of  the  "  hooped  column,"  that  the  horizontal  metal 
jacket  prevents  the  concrete  from  spreading  and  thereby  in- 
creases its  carrying  capacity.1 

For  practical  reasons  it  has  been  found  impossible  to  use  a 
continuous  sheet  of  iron  around  the  concrete;  the  horizontal 
reinforcement  is  always  in  the  shape  of  hoops  encircling  the  body 
of  the  concrete.  Under  pressure  the  concrete  is  sometimes  seen  to 
ooze  out  between  the  hoops,  indicating  the  failure  of  the  column, 
but  usually  the  column  fails  by  the  bursting  of  the  hoops  or  the 
complete  disintegration  of  the  concrete.  In  practical  construc- 
tion this  need  not  concern  us,  as  the  stresses  naturally  always  are 
low;  more 'important  is  the  relatively  great  shortening  of  the 
hooped  column  under  working  loads.  This  objection  is  overcome 
by  the  rational  use  of  vertical  rods,  so  that  the  true  "  hooped 
column  "  contains  both  hoops  and  verticals  (Figure  81) 


FIGURE  81. 

13.  The  computation  of  a  hooped  column  naturally  centers 
around  the  calculation  of  the  lateral  pressure  against  the  hoops. 
With  a  given  concrete  area  F  and  a  given  load  X  the  unit  stress 
on  the  concrete  becomes 

^  IK    /      •«  K  5  wnere  -X"  is  m  Ibs. 

F  lbs'/Sq-  mch  (and  F  in  square  inches. 

If  we  were  dealing  with  water,  the  horizontal  unit  pressure  would 
be  the  same.     For  concrete  this  is  not  the  case;    according  to 

1  Attention  is  called  to  some  very  interesting  tests  by  Prof.  Ira  H.  Wool- 
son,  Eng.  News,  1905,  Nov.  2,  —  Steel  tubes,  4"  in  diameter  and  12"  long, 
| "  thick  walls,  were  filled  with  concrete.  When  seventeen  days  old,  the  tubes 
were  tested  in  compression  under  loads  as  high  as  120,000  to  150,000  Ibs. 
The  tubes  bent  out  of  shape,  and  shortened  3£",  while  the  diameter  increased 
from  the  original  4"  to  5".  When  the  tubes  were  removed,  the  concrete  was 
found  unbroken,  solid,  and  perfect. 

See  also  Trautwine,  1909,  p.  1160. 


60  REINFORCED  CONCRETE  BUILDINGS 

experiment,  the  ratio  between  intensity  of  vertical  stress  and 
transverse  stress  is  as  1  to  1/4.8.  In  other  words,  if  the  load 
produces  a  direct  compressive  stress  of  4801bs./sq.  inch,  the  lateral 
pressure  would  at  the  same  time  be  100  Ibs./sq.  inch.  It  is  now 
a  simple  matter  to  write  up  an  expression  giving  the  resistance 
due  to  the  hoops,  in  a  granular  material  having  this  same  coeffi- 
cient 4.8.  Let  us  denote  by  u  the  ratio  between  this  resistance 
and  the  volume  of  metal  in  the  hoops,  and  let  us  denote  by  U  a 
similar  ratio  obtained  between  the  resistance  due  to  vertical 
reinforcement,  and  the  volume  of  the  material  in  the  verticals. 
The  expressions  u  and  U  will  then  give  the  effect  produced  by  a 
unit  of  material,  used  as  hoops  and  as  verticals.  We  find, 
assuming  the  same  stress  in  hoops  and  verticals: 

^-M_24 
U       2 

which'  shows  that  pound  for  pound,  the  steel  employed  in  the 
hoops  is  2.4  times  as  effective  as  steel  employed  for  longitudinal 
reinforcement. 

14.  The  question  is  now  to  find  the  effect  of  the  verticals. 
Assuming  that  they  are  well  tied  so  as  to  prevent  buckling  of  the 
individual  rods,  the  unit  stress  on  the  verticals  must  be  r  times 
the  stress  on  the  concrete,  if  the  sections  are  to  remain  plane  as 
assumed  in  Article  4.  It  is  easy  to  see  that  this  assumption  is 
on  the  safe  side,  because,  if  the  sections  curved,  the  stress  in  the 
steel  might  be  very  much  more  than  r  times  that  on  the  con- 
crete, which  latter  forms  the  starting-point  for  our  investigation. 
The  value  of  r  can  only  be  indicated  in  a  general  way,  as  the 
properties  of  concrete  vary  greatly  with  the  circumstances;  let 
us  assume  r  =  20.  Then,  if  the  unit  stress  on  the  concrete  is 
500  Ibs./sq.  inch,  the  stress  on  the  steel  becomes  10,000  Ibs./sq. 
inch.  Let  F  be  the  area  of  the  concrete  inside  the  hoops,  and  the 
allowable  stress  on  this  concrete  C  Ibs./sq.  inch.  Let  p  denote 
the  percentage  of  the  verticals  with  reference  to  the  volume  of 

concrete,  then  the  effective  concrete  area  is  F-  - 

1UU 
7? 

and  the  area  of  the  longitudinals  F  •  T^TT  ; 

hence  the  load  carried  by  the  concrete  is  C  -  F fnf)      ^s. 


COMPRESSION  AND  LATERAL  EXPANSION  61 

Let  the  stress  on  the  longitudinals  be  S  Ibs./sq.  inch,  then  their 

7) 

share  of  the  load  becomes  S  •  F  •  -      Ibs. 


Disregarding  for  the  moment  the  influence  of  the  hoops,  the  total 
carrying  capacity  of  the  reinforced  column  is 


while  if  allowance  be  made  for  the  hoops,  the  percentage  of  which 
is  q  with  reference  to  the  concrete  section,  we  have  an  additional 

strength  due  to  the  hoops  equal  to  2.4  •  S  •  F  •  -^ 
and  the  total  carrying  capacity  of  the  column  becomes 

.       (2) 


15.  The  formula  (2)  above  is  the  true  formula  for  a  reinforced 
concrete  column  and  should  always  be  used  except  in  localities 
where  the  building  code  prevents  its  use,  in  which  case  formula 
(1)  may  be  used.  In  any  case,  hoops  must  be  used,  otherwise 
the  column  steel  is  of  no  value  as  reinforcement.  For  the  hooped 
column,  Considere,  the  inventor  and  first  experimenter,  recom- 
mends p  =  q  =  2,  which,  with  C  =  600  Ibs./sq.  inch,  and  r  =  20, 
gives 

X2  =  1400  •  F. 

The  hoops  are  spaced  as  closely  as  possible,  leaving  1"  to  2" 
clear  space  between  the  hoops  to  facilitate  concreting.  The 
spacing  should  under  no  circumstances  exceed  1/6  of  the  diam- 
eter of  the  core.  Finally  the  core  is  protected  with  a  suffi- 
cient thickness  of  concrete  to  prevent  rust  and  fire  danger,  about 
1  "  to  2  "  of  protection  being  required  according  to  location  and 
exposure. 

The  plain  column  has  a  vertical  reinforcement  varying  from 
one  to  ten  per  cent,  of  the  concrete  area,  although  reinforcement 
in  excess  of  say  five  per  cent,  should  be  avoided  on  account  of 
the  uncertainty  of  the  strength  of  columns  reinforced  with  large 
amounts  of  steel.  It  is  evident  that  hoops  are  indispensable  also 
in  these  columns;  it  is  quite  common  to  see  the  hoops  spaced 
one  or  even  two  feet  apart;  such  hoops  are  of  no  use.  The 
steel  cannot  be  depended  upon  to  carry  its  load  unless  securely 


62  REINFORCED  CONCRETE  BUILDINGS 

tied,   say,    1/3   to   1/2   column  diameters  apart.     With   p  =  4, 
8  =  12,000,  C  =  600,  we  have,  for  r  =  20: 

Xl  =  1060  •  F. 

16.  Owing  to  difficulties  in  filling  columns  of  small  diameter, 
the  diameter  should  not  be  much  less  than  10 "  in  any  case, 
although  there  are  many  8"  columns  on  record.     On  account  of 
the  danger  of  "  column  failure  "  the  length  should  not  exceed 
15  diameters.     It  is  possible  to  advance  a  theory  for  "  long  " 
columns,  but  experience  shows  that  columns  exceeding  15  diam- 
eters in  length  are  rare  indeed  except  in  roof  stories  where  the  calcu- 
lations often  give  very  light  sections.     Moreover,  all  such  theories 
depend  alone  upon  theoretical  considerations  and  have  never  been 
conclusively  tested  in  the  laboratory,  so  that  in  the  rare  cases 
where  "  long  "  columns  are  required  it  is  better  to  make  the  col- 
umn a  little  larger  and  avoid  the  uncertainties  of  the  theory. 

17.  In  tall  buildings,  or  in  warehouses,  the  column  bars  become 
quite  heavy,  and  it  is  necessary  to  join  the  bars  of  the  column 
above  with  those  of  the  column  below  in  a  substantial  manner. 
The  most  satisfactory  way  is  to  square  the  ends  of  the  bars 
carefully  and  join  them  in  rather  closely  fitting  sleeves,  taking 
care  that  each  bar  has  a  full  bearing  on  the  bar  below.     Absolute 
certainty  is  had  by  cutting  threads  on  both  bars  and  sleeves,  and 
drawing  the  bars  together  tight  with  the  sleeve,  but  this  must 
be  done  with  great  care  and  under  strict  supervision  in  order  to 
be  at  all  effective;  —  unless  carefully  made  this  joint  is  worse 
than  useless.     When  light  bars  are  used  they  may  be  spliced  by 
lapping  by  the  required  number  of  diameters,  say  about  thirty, 
but  this  method  is  hardly  to  be  recommended. 

Each  bar  of  the  story  above  should  find  bearing  on  a  bar 
below;  the  number  of  bars  therefore  increases  downward  in  the 
building.  The  number  of  bars  in  each  story  should  be  such  that 
the  bars  can  be  symmetrically  arranged  in  the  column,  unless 
there  is  some  extraordinary  reason  for  arranging  them  otherwise 
(excentric  loads).  The  proper  arrangement  of  the  column  bars 
may  sometimes  cause  the  designer  to  spend  a  good  deal  of  time  in 
working  out  the  correct  solution,  but  he  may  feel  assured  that 
this  time  is  well  spent. 

The  hoops  may  be  made  from  round  or  flat  stock;  the  round 
stock  may  be  obtained  in  long  lengths  and  lends  itself  more  readily 
to  the  requirements  of  the  hooped  column,  especially  where  the 


COMPRESSION   AND   LATERAL  EXPANSION 


63 


reinforcement  is  manufactured  in  the  shop,  with  permanent 
devices  for  coiling  and  fastening  the  hoops  to  the  longitudinals. 
The  hooped  reinforcement  may  also  be  bought  ready-made; 
quite  frequently  the  manufacturer  overlooks  the  importance  of 
having  the  spiral  hoops  in  one  continuous  piece  from  top  to 


FIGURE  82.  COLUMN  REINFORCEMENT 

Loomis  Building,  Cleveland,  Ohio.     Alexis  Saurbrey,  Consulting 
Engineer 

bottom,  or,  where  the  wire  is  joined,  he  makes  a  flimsy  joint. 
It  must  be  remembered  that  the  hoops  are  tension-reinforcement 
and  subject  to  all  the  rules  governing  the  design  of  such  bars. 
The  best  joint  is  made  by  simply  bending  the  ends  of  the  wire 
to  the  center  of  the  column,  making  the  loose  end  long  enough 
to  secure  the  requisite  grip. 

The  hoops  may  also  be  made  in  individual  pieces,  slipped 
over  the  previously  erected  verticals  and  wired  in  place.  If 
the  hoops  are  neatly  made  an  excellent  job  may  be  had  in  this 
way  (Figure  82). 


64  REINFORCED  CONCRETE  BUILDINGS 

18.  It  follows  from  what  is  said  above  that  a  hooped  column 
should  preferably  be  made  of  a  circular  cross-section,  because 
in  that  case  the  hoops  are  subject  to  direct  tension  only.     In 
many  cases  the  expense  incidental  to  the  use  of  circular  forms  is 
prohibitive;   the  concrete  may  then  be  made  square  or  octagon 
in  section  while  the  circular  form  is  retained  for  the  hoops.     In 
either  case  only  the  concrete  within  the  hoops  can  be  taken  into 
account  in  the  calculations.     Sometimes  the  hoops  are  made 
square  or  rectangular,  in  which  case  they  are  less  effective,  but 
we  do  not  know  how  much. 

19.  The  top  and  bottom  of  each   column  deserves  special 
attention  as  the  tests  made  so  far  seem  to  indicate  that  these  are 
the  weakest  parts  of  the  column,  although  there  are  many  ex- 
ceptions to  this  rule.     Suitable  caps  and  bases  are  inexpensive, 
improve   the   appearance   and   increase   the   strength.     Special 
investigation  is  always  necessary  at  points  where  the  concrete 
column  finds  a  bearing  on  another  material;   the  weight  carried 
by  the  reinforcing  rods  must  be  distributed  over  such  an  area 
that  the  concrete  in  the  column  is  not  over-stressed.     This  is 
particularly  true  where  the  column  rests  on  the  footing;   a  steel 
base  plate  must  be  used  to  distribute  the  load  on  the  rods,  and 
the  concrete  must  be  enlarged  so  as  to  bring  the  average  pressure 
within  the  allowable.     This  will  be  considered  in  detail  under 
"footings." 

20.  Before  leaving  the  subject  of  hooped  columns,  attention 
is  called  to  the  possibility  of  strengthening  existing  concrete 
columns  with  hoops  wound  around  the  outside  of  the  column. 
In  many  cases  it  would  be  impossible  to  obtain  satisfactory  results 
in  this  manner,  but  when  the  concrete  is  of  good  quality,  and  the 
existing  reinforcement  is  such  as  to  give  a  sufficient  amount  of 
longitudinal  reinforcement  in  the  finished  column,  there  should 
be  no  theoretical  objections  to  this  procedure.     In  practice  it 
would  of  course  be  difficult  to  wrap  the  core  tightly,  but  this  is 
not  absolutely  necessary,  as  grout  rich  in  cement  may  be  forced 
between  the  hoops  and  the  old  concrete.     Great  care  would  be 
required  in  this  operation,  but  it  is  not  at  all  impossible,  as  has 
been  shown  by  actual  experiments  on  a  small  scale.1 

1  Considere:  "Reinforced  Concrete,"  page  175.  The  prism  tested  in 
this  manner  was  allowed  to  set  for  three  months,  then  wrapped  with  hoops 
and  covered  with  cement,  and  tested  after  ten  more  days.  The  crushing 


COMPRESSION  AND  LATERAL  EXPANSION  -  65 

21.  In  many  cases  columns  are  subject  to  excentric  loads,  so 
that,  in  addition  to  the  direct  compressive  force,  a  bending 
moment  exists  and  must  be  taken  care  of.  This  will  be  con- 
sidered in  detail  in  Article  81. 

strength  was  10,500  Ibs.  per  square  inch.     There  were  no  longitudinals  in 
this  prism. 


CL  AFTER  VI 
BENDING 

22.  THE  theory  of  bending  used  for  reinforced  concrete  beams 
is  different  from  the  ordinary  "  theory  of  flexure  "  as  used  for 
homogeneous  beams  in  a  few  particulars  only,  and  this  difference 
is  more  apparent  than  real.  We  consider  here  only  the  point  of 
maximum  bending  moment;  this  is  also  the  point  of  maximum 
depth,  and  we  may  assume  both  the  compressive  and  tensile 
resultant  to  be  normal  to  a  vertical  section  through  this  particular 
point,  under  the  particular  loading  described  below. 

The  notations  used  are  as  follows  (Figures  83,  84) : 

d  or  D  =  depth  from  top  of  concrete  to  center  of   steel, 

inches. 
xd  =  depth  from  top  of  concrete  to  neutral  axis,  inches. 

xd 
x  =~j~=  ratio  between  the  two  preceding  items. 

di  =  distance  center  of  compression  to  center  of  ten- 
sion, inches. 
Ec  and  Es  =  coefficients  of  elasticity  for  concrete  and  steel. 

Tjl 

r  =  -pr-=  ratio  between  these  coefficients. 

tic 

t  or  T  =  thickness  of  a  flange,  inches. 

5  =  width  of  flange  considered,  inches. 

B  =  ^iS  =  width  of  flange  considered,  feet. 
\2i 

n  =  thickness  of  stem  of  beam,  inches. 
rc  and  rs  =  deformations  of  concrete  and  steel,  at   extreme 

fiber. 

C  =  unit  stress  on  concrete  in  outside  fiber,  compres- 
sion, Ibs.  per  square  inch. 

S  =  unit  stress  in  steel,  tension,  Ibs.  per  square  inch. 
a  =  area  of  steel,  square  inches. 
St.  =  total  pull  in  steel  in  tons. 
Clj  c2,  c3  =  coefficients  relating  to   balanced   design   of   the 

section. 

a,  £  =  coefficients  relating  to  T-beams  with  greater  than 
minimum  depth. 
66 


BENDING 


67 


w  =  dead  plus  live  load  on.  slab,  Ibs.  per  square  foot. 
/  =  span  in  feet. 
q  =  factor  of  continuity. 
M  =  bending  moment  in  tons-inches. 
m  =  2000  M  =  bending  moment  in  Ibs.-inches. 

23.  In  regard  to  the  load,  we  wi^  let  all  loads  act  in  the  same 
vertical  plane  along  the  center  line  of  the  beam  as  is  usually  the 
case  in  practical  construction.     This  excludes  at  once  all  loads 
which  would  cause  the  beam  to  rotate  around  its  longitudinal 
axis  and  all  loads  which  would  cause  the  beam  to  slide  in  its  own 
direction. 

24.  In  regard  to  the  deformations,  we  will  consider  these  as 
very  small  in  comparison  with  the  dimensions  of  the  beam,  so 
that  the  stresses  are  considered  as  acting  upon  the  original  cross- 
sections,   not   upon   the   deformed   cross-sections   or  upon   the 
deflected  beam. 

25.  This  does  not  mean  that  the  change  of  shape  of  the  section 
is  of  no  importance.     In  figure  83  a  vertical  section  is  shown 


\  Shortening 
;=  <  of  Top  Fibre, 
*  Concrete 


Elongatio 
|     of~Steel 

FIGURE  83. 


with  the  deformations  produced  by  the  bending  of  the  beam; 
we  assume  sections  plane  before  bending  to  remain  plane  after 
bending.  Inspection  of  the  diagram  shows  that  the  upper  fibers 
are  shortened,  the  lower  fibers  extended  under  the  load;  —  the 
neutral  axis  forms  the  division  line  between  shortened  and  ex- 
tended fibers.  The  assumption  of  plane  sections  is  evidently 
equivalent  to  assuming  that  the  deformation  of  any  fiber  is  in 


68  REINFORCED  CONCRETE  BUILDINGS 

direct  proportion  to  its  distance  from  the  neutral  axis,  and  thus 
we  get  the  equation: 

r-<  =  **—  =  _^_  ,3) 

ra        (l-x)d        l-x 

26.  We  further  assume  that  the  stress  on  any  small  unit  is 
directly  proportional  with  the  deformation;  this  gives  the  equa- 
tions : 

Q 

for  concrete     C  =  rcEc  or  rc  =  ^r 


o 

for  steel  S  =  r8Es  or  rs  =  7 


___ 

TS          S'  EC 


(4) 


27.  We  shall  later1  have  occasion  to  use  the  moment  of  inertia 
of  the  section.     It  is  therefore  necessary  to  note  that  the  assump- 
tions made  in  the  preceding  paragraphs  are  identically  the  same 
as  those  used  in  the  "  common  theory  of  flexure  "  which  leads 
to  the  well-known  expression 

a  _  ^  .  e  where  v  =  stress  per  unit  (5) 

I  M  =  bending  moment 

/    =  moment  of  inertia 
e    =  distance  from  neutral  axis  to  fiber 
considered. 

The  new  feature  in  a  reinforced  concrete  beam  is  now  that  in 
writing  up  the  moment  of  inertia  we  have  to  disregard  the  con- 
crete below  the  neutral  axis  entirely,  and  instead  consider  the 
steel  area.  To  this  we  shall  return  later. 

28.  Combining  now  equations  3  and  4  we  find 


S      Ec 

hence  x 


which  expression  determines  the  location  of  the  neutral  axis. 

29.  If  now  a  vertical  section  is  laid  across  the  beam  and 
stresses  added  on  and  in  the  section  to  represent  the  removed 
portion  of  the  beam,  the  beam  will  remain  in  equilibrium.  Let 
us  project  all  forces  and  stresses  on  a  horizontal  line:  then  the 

1  Article  79. 


BENDING  69 

loads,  being  vertical,  give  no  projections,  and  similarly  the 
stresses  acting  in  the  vertical  section  itself  disappear.  There 
remain  only  the  normal  stresses  acting  against  the  section;  as 
equilibrium  presupposes  that  the  sum  of  all  the  projected  forces 
and  stresses  is  zero,  we  have 


horizontal  component 

of  stresses 
on  tension  side 


horizontal  component 

of  stresses 
on  compression  side. 


Referring  now  to  Figure  84,  the  area  stressed  in  compression  is 
xd  inches  high,  b  inches  wide,  and  the  average  stress  J  C  Ibs.  per 
square  inch.  Hence 

total  compression  =  \  C  •  xd  •  b  Ibs. 

Denoting  by  st  the  total  pull  in  the  steel  in  tons,  we  have,  neglect- 
ing the  tension  in  the  concrete, 

total  tension  =  st  •  2000  Ibs. 

Hence    st  •  2000  =  i  Cxdb 

,  .  ,      .  Cxdb  c2  ,  , 

which  gives  st  =  TTTT    or     st  =  Jo" 


where  c2  =   -  (6) 


30.  Two  more  conditions  must  be  fulfilled  in  order  to  create 
equilibrium:  (1)  the  sum  of  all  stresses  and  forces  must  be  zero 
when  projected  upon  a  vertical  line  (when  the  loads  are  vertical, 
Article  23);  this  condition  we  will  consider  later  under  "  U-bars." 
(2)  The  sum  of  all  moments  around  any  arbitrary  point  must  be 
zero.  Select  for  this  point  the  point  of  application  of  the  com- 
pressive  stresses;  the  moment  of  the  loads  is  then  the  "  bending 
moment  "  m  inch-lbs.  The  moment  of  the  stresses  is  2000  st  d\ 
inch-lbs.  We  must  then  have 

0  =  m  -  2000  st  .  di 

but  according  to  the  diagram  (Figure  84) 

d,  =  (1  -  J  x)  d 
hence  0  =  m  -  2000  •  st'  (1  -  J  x)  d. 

Eliminating  st  we  find 

m  =  -I  Cxb  (1  -  J  x)  d* 


hence  d  =  -  V        inches  where    Ci  =  V  I  Cx  (1  —  -|  x)  (7) 

c\  •    o 

Finally  the  steel  area:    a  =  —  —  st  square  inches. 


70 


REINFORCED  CONCRETE  BUILDINGS 


31.  The  formulas  apply  to  all  rectangular  beams  and  therefore 
also  to  slabs.     As  we  disregard  the  tensile  resistance  of  the  con- 
crete, the  concrete  below  the  neutral  axis  does  not  in  any  way 
enter  into  the  calculations  at  this  point,  and  the  formulas  are 
therefore  also  correct  for  T-beams  where  the  bottom  of  the  flange* 
coincides  with  the  neutral  axis.     In  this  case  the  thickness  of 
flange  simply  becomes 

t  =  xd  inches. 

32.  We  have  now  everything  required  to  proceed  with  the 
design: 


StTons 


FIGURE  84. 


The  depth  in  inches: 

The  pull  in  the  steel,  tons: 


*~^T 


a  = 


The  thickness  of  flange,  inches:    t  =  xd 

2000 
Ihe  steel  area,  square  inches:     a  = 

o 


(8) 

(9) 
(10) 
(11) 


Simple  as  these  formulas  are  they  can  only  be  used  when  the 
values  of  the  coefficients  x,  ci  and  c2  are  known,  and  these  values 
in  turn  depend  upon  the  allowable  stresses  and  the  factor  r. 
The  Tables  I,  II,  and  III  give  full  information  in  regard  to  the 
values  of  the  coefficients;  it  will  be  noticed  that  the  same  tables 
may  be  used  for  any  value  of  r,  by  simply  shifting  the  position 
of  the  S-column  in  relation  to  the  values  of  the  coefficients.  On 
the  left  the  ordinarily  used  ^-column  is  indicated,  corresponding 
to  r  =  15;  while  on  the  right,  the  ^-columns  corresponding  to 
r  =  12  and  r  =  20  are  showrn.  Usually  existing  building  codes  and 
engineers'  specifications  call  forr  =  15  in  bending-problems,  but 


BENDING  71 

this  selection  is  arbitrary,  and  other  values  of  r  may  very  well 
be  used.  It  is  impossible  to  predict  the  coefficient  of  elasticity 
of  concrete  beforehand,  and  even  if  determined  by  careful  ex- 
periment there  is  no  reason  to  believe  that  it  would  remain  the 
same  on  the  building  to  be  erected  as  in  the  laboratory,  while  it 
is  quite  certain  that  it  changes  materially  from  day  to  day  as 
temperature  and  moisture  affect  the  mixture  used  for  the 
concrete. 

In  Table  IV  values  of  the  coefficient  c3  =  1  —  i  x  are  indicated; 
the  use  of  this  table  will  be  clear  from  the  analysis  above.  In 
Table  V  the  percentage  of  steel  in  a  rectangular  beam  is  indicated 
corresponding  to  r  =  15;  when  the  allowable  stresses  are  decided 
upon,  the  percentage  of  steel  in  the  section  is  a  fixed  quantity. 

33.  In  the  formulas  above  all  dimensions  are  in  inches,  the 
moment  in  inch-lbs.,  the  pull  st  in  tons.     In  practical  design  it  is 
usually  convenient  to  have  the  bending  moment  in  inch-tons,  M, 
and  the  width  in  feet,  B.     The  formulas  then  become : 

The  depth  in  inches :  d  =  —   —  •  y  -^-        (8a) 

The  pull  in  the  steel,  tons:          st  =  c%Bd ;  (9a) 

The  thickness  of  flange,  inches:    t  =  xd  (10a) 

2000 
The  steel  area,  square  inches:      a  =  — ~—  Si  (11) 

These  formulas  are  different  from  those  given  above  in  this 
respect  only,  that  the  figures  handled  are  much  smaller  and  there- 
fore it  becomes  easier  to  avoid  mistakes,  as  figures  of  two  or  three 
places  may  be  multiplied  and  divided,  etc.,  approximately,  without 
the  use  of  paper  and  pencil,  so  that  all  calculations  are  easily 
verified. 

34.  The  formulas  given  above  apply,  as  stated,  to  slabs,  to 
rectangular  beams,  and  to  T-beams  in  which  the  neutral  axis 
coincides  with  the  bottom  line  of  the  flange.      Usually  these 
two  lines  do  not  coincide,  so  that  it  becomes  necessary  to  make 
further  investigation  in  order  to  derive  a  general  formula.     The 
formulas  given  above  have  this  peculiarity,  that,  for  a  given  width 
of  beam,  the  dimensions  derived  are  minimum  dimensions  which 
cannot  be  decreased  without  adding  to  the  stress  on  the  material, 
thus  exceeding  the  allowable  stresses  on  which  the  design  was 
based.     Briefly  stated,  the  problem  before  us  consists  in  finding 


72 


REINFORCED  CONCRETE  BUILDINGS 


TABLE   I.     DEPTH  OF  NEUTRAL  Axis  =  xd 

1 


r  =  15 

TABLE  I    x 

r  =  12 

r  =20 

S  =  24,000 

.158 

.200 

.238 

.272 

.304 

.333 

.360 

.385 

S  =  19,200 



22,000 

.170 

.214 

.254 

.290 

.322 

.352 

.380 

.405 

17,600 



20,000 

.184 

.231 

.272 

.310 

.344 

.375 

.404 

.429 

16,000 

— 

18,000 

.200 

.250 

.294 

.333 

.369 

.400 

.429 

.454 

14,400 

— 

16,000 

.219 

.272 

.318 

.360 

.397 

.429 

.458 

.483 

12,800 

S  =  24,000 

14,000 

.244 

.300 

.349 

.392 

.429 

.463 

.491 

.519 

11,200 

21,300 

12,000 

.272 

.333 

.385 

.429 

.468 

.500 

.529 

.556 

9,600 

18,600 

10,000 

.310 

.376 

.429 

.474 

.513 

.546 

.574 

.602 

— 

16,000 

C  = 

300 

400 

500 

600 

700 

800 

900 

1000 

r  =  15 

r  =  12 

r=  20 

TABLE   II.     EFFECTIVE  DEPTH 

12.9     /"M  1        /m 

d  =  4/— or  d  =  ->.      - 

ci  y  B         Cl  y  6 


r  =  15 

TABLE  II  ci 

r  =  12 

r  =  20 

S  =  24,000 

4.7 

6.1 

7.4 

8.6 

9.8 

11.0 

12.0 

13.0 

-S  =  19,200 

_ 

22,000 

4.9 

6.3 

7.6 

8.8 

10.0 

11.2 

12.2 

13.2 

17,600 

— 

20,000 

5.1 

6.5 

7.9 

9.1 

10.3 

11.5 

12.5 

13.5 

16,000 

— 

18,000 

5.3 

6.8 

8.1 

9.4 

10.6 

11.8 

12.8 

13.9 

14,400 

— 

16,000 

5.5 

7.0 

8.4 

9.7 

11.0 

12.1 

13.2 

14.2 

12,800 

S  =  24,000 

14,000 

5.8 

7.3 

8.8 

10.1 

11.3 

12.5 

13.6 

14.7 

11,200 

21,300 

12,000 

6.1 

7.7 

9.2 

10.5 

11.7 

12.9 

14.0 

15.1 

9,600 

18,600 

10,000 

6.5 

8.2 

9.6 

11.0 

12.2 

13.4 

14.5 

15.5 

— 

16,000 

C  = 

300 

400 

500 

600 

700 

800 

900 

1000 

— 



r  =  15 

r  =  12 

r  =20 

TABLE  III.     TOTAL  PULL  IN  STEEL 


s.  =  02  Bd  or  sf  =  —  02  6d 
«  *       12 


r  =  15 

TABLE  III  c2 

r  =  12 

r  =  20 

S  =  24:000 

.14 

.24 

.36 

.49 

.64 

.80 

.97 

1.16 

S  =  19,200 



22,000 

.15 

.26 

.38 

.52 

.68 

.85 

1.03 

1.22 

17,600 

— 

20,000 

.17 

.28 

.41 

.56 

.72 

.90 

1.10 

1.29 

16,000 

— 

18,000 

.IS 

.30 

.44 

.60 

.78 

.96 

1.16 

1.36 

14,400 

— 

16,000 

.20 

.33 

.48 

.65 

.83 

1.03 

1.24 

1.45 

12,800 

S  =  24,000 

14,000 

.22 

.36 

.52 

.71 

.90 

1.12 

1.33 

1.56 

11,200 

21,300 

12000 

.25 

.40 

.58 

.77 

.99 

1.20 

1.43 

1.67 

9,600 

18.600 

10,000 

.28 

.45 

.64 

.86 

1.08 

1.31 

1.55 

1.80 

— 

16,000 

C  = 

300 

400 

500 

600 

700 

800 

900 

1000 

— 

— 

r  =  15 

r  =  12 

r  =  20 

BENDING 


73 


TABLE   IV.     ARM  OF  "COUPLE  OF  STRESSES." 


r  =  15 

TABLE  IV  c3  =  1  -  \x 

r  =  12 

r  =  20 

S  =  24,000 

.95 

.93 

.92 

.91 

.90 

.89 

.88 

.87 

5  =  19,200 

— 

22,000 

.94 

.93 

.92 

.90 

.89 

.88 

.87 

.87 

17,600 

— 

20,000 

.94 

.92 

.91 

.90 

.89 

.88 

.87 

.86 

16,000 

— 

18,000 

.93 

.92 

.90 

.89 

.88 

.87 

.86 

.85 

14,400 

— 

16,000 

.93 

.91 

.89 

.88 

.87 

.86 

.85 

.84 

12,800 

S  =  24,000 

14,000 

.92 

.90 

.88 

.87 

.86 

.85 

.84 

.83 

11,200 

21,300 

12,000 

.91 

.89 

.87 

.86 

.84 

.83 

.82 

.82 

9,600 

18,600 

10,000 

.90 

.88 

.86 

.84 

.83 

.82 

.81 

.80 

— 

16,000 

c  = 

300 

400 

500 

600 

700 

800 

900 

1000 

— 

— 

r  =  15 

r  =  12 

r  =  20 

Amount  of  Steel  In  Section 

P— i*T  *  100 
-b  =12 


TABLE  V 


r  =  15 

p 

S  =  24,000 

.098 

.167 

.247 

.339 

.442 

.553 

.672 

.801 

22,000 

.115 

.196 

.288 

.393 

.510 

.636 

.771 

.920 

20,000 

.138 

.231 

.339 

.465 

.602 

.750 

.907 

1.07 

18,000 

.166 

.278 

.406 

.553 

.714 

.884 

1.07 

1.26 

16,000 

.204 

.339 

.493 

.672 

.865 

1.07 

1.27 

1.50 

14,000 

.261 

.428 

.621 

.839 

1.07 

1.33 

1.58 

1.85 

12,000 

.339 

.554 

.799 

1.07 

1.36 

1.66 

1.98 

2.31 

10,000 

.463 

.750 

1.07 

1.42 

1.79 

2.17 

2.57 

2.99 

C  = 

300 

400 

500 

600 

700 

800 

900 

1000 

the  effect  on  the  T-beam  of  an  increase  in  depth,  which  must,  in 
order  to  balance  the  design,  be  accompanied  by  a  corresponding 
decrease  in  thickness  of  flange  and  amount  of  steel. 


74 


REINFORCED  CONCRETE  BUILDINGS 


35.  Let,  then,  Figure  85a  represent  a  section  of  T-shape  of 
minimum  dimensions,  having  a  depth  d,  a  thickness  of  flange 
ta,  and  a  total  pull  in  the  steel  of  sa  tons.  Let,  further,  Figure 
856  represent  a  new  section  with  a  new,  larger  depth  D  =  ad. 


b=12B 

ta 

=  xd 

/Neutral  Axis 

\ 

<:  n  > 

c 

9 

* — r 


ad= 


/Neutral  Axis      | 


FIGURE   85a. 


FIGURE  856. 


The  given  M  and  B  remain  the  same;  we  wish  to  determine  the 
new  values  sb  and  T  pertaining  to  Figure  856. 

We  observe,  then,  that  the  proportionate  depth  x  of  the  neutral 
axis  is  the  same  in  the  two  beams,  because  the  allowable  stresses 
are  the  same,  so  that  the  depth  of  neutral  axis  is  calculated  as 
ta  =  xd  in  the  first  beam  and  as  tb  =  xD  in  the  second.  The 
"  effective  depth"  in  the  first  beam  is 

di  =  (1  —  |  x)  d 
and  in  the  second  approximately 

Di  =   (1  -  %X)  D  =    (1  -  J  X)  ad 

The  approximation  consists  in  disregarding  the  tendency  of  the 
center  of  compression  to  rise  on  account  of  the  removal  of  the 
concrete  near  the  neutral  axis;  the  discrepancy  is  negligible  in 
most  cases  and  on  the  safe  side.  Since  now 


M         , 

--    and 


M 


we  get  the  equation  sa  =  t*sb  where  sa  = 
and,  by  reference  to  Figure  86 

T-6  + 


4000 


CT 


4000 

Introducing  these  values  in  sa 
an  equation 


(at  -  T)  -  n. 


4000 

sb  we  get  after  some  reduction 


BENDING 


75 


ST\ 

Solving  for  (  —  )  and  denoting  by 

\t  a  J 

p—  b~H  i      t 

a2    1  r  ]     '  ta  •  r 

\            ata/                   0 

ft  the  value  of  this  ratio  we  find 

n                                 (L^} 
b' 

1  3FC  |-          °K"~ 

*>*^ 

s 

Neutral  Axis 

-Jn  <&- 

1  • 

^k 

C   =r  ata-T       j 

°T     C~^T-      | 

FIGURE  86. 
Showing  the  stresses  in  the  beam  of  Figure  856. 


The  thickness  of  flange  in  our  new  beam  is  now 
T  =  pta  =  Pxd  =  --x-D 

a. 

and  the  new  total  pull  in  the  steel  is 


a.         a  a* 

corresponding  to  the  new  depth  D  =  ad. 

36.    We  have  now  the  following  general  formulas  for  any  re- 
inforced concrete  T-section: 

12.9       I'M 


The  depth,  in  inches 


D  =  a. 


IM 

VB 


The  total  pull  in  the  steel,  in  tons     s<  =  -| 


The  thickness  of  flange,  in  inches 
The  steel  area 


a  = 


2000 


(13) 


(14) 
(15) 


s( 


where  M  is  the  bending  moment  in  tons-inches,  a  an  arbitrary 
coefficient  larger  than  unit,  while  the  width  B  feet  may  be  given 
or  selected.  The  coefficient  ft  is  derived  from  «  by  formula  12 
but  to  facilitate  calculations,  Table  VI  has  been  prepared  giving 
the  values  of  fi  for  various  combinations  of  a  and  n/b.  This 
latter  ratio  has  little  influence  on  the  result  within  the  ordinary 
limits,  and  Table  IX  may  also  be  used  in  cases  where  n/b  is 


76 


REINFORCED  CONCRETE  BUILDINGS 


different  from  1/4,  if  the  variation  is  not  too  large,  although 
prepared  especially  for  n/b  =  1/4. 

37.  The  theory  of  T-beams  is  of  great  importance  as  all  the 
floor  systems  in  common  use  involve  this  principle.  Lately, 
beamless  floors  have  come  into  use,  and  to  these  we  shall  return 
later;  the  beam  and  slab  floors  may  be  divided  into  two  groups, 
the  first  including  solid  concrete  floors,  the  second  what  is  known 
as  "  tile-concrete "  floors.  The  first  of  these  two  is  by  far  the 
oldest,  but  the  "  tile-concrete "  is  gaining  in  favor  with  every 
day,  and  justly  so,  as  its  cost  is  less  for  light  buildings  owing 
primarily  to  the  simplicity  of  the  form  work.  The  long  flat 
ceilings  are  well  adapted  to  modern  store  building  and  office- 
structures,  especially  where  the  loads  are  light  and  distributed. 
These  floors  have  a  flat  portion  supported  on  main  girders  A 
(Figure  87),  the  flat  portion  consisting  of  ribs  B  built  between 


I  c  I  cTc  I I       I 


Section  A-A 

FIGURE  87. 

rows  of  hollow  tiles  C  and  a  top  covering  of  two  or  more  inches  of 
concrete,  thus  forming  series  of  comparatively  light  T-beams 
side  by  side.  The  main  girders  are  also  of  T-shape,  the  flanges 
being  formed  by  leaving  out  the  requisite  number  of  tiles  next 
to  the  stem  of  the  girder.  Sometimes  lighter  tiles  D  are  used 
near  the  stem,  in  which  case  the  flange  becomes  thinner  than  when 
the  tiles  are  omitted  entirely,  Figure  88.  The  commercial  sizes 


FIGURE  88. 


of  tiles  are  usually  12"  x  12"  in  plan,  the  depth  ranging  from  4" 
to  12"  or  even  16".  When  designing,  it  becomes  necessary  to 
proportion  the  depth  of  floor  so  as  to  allow  for  these  commercial 


BENDING  77 

sizes;  the  function  of  the  tiles  is  simply  to  create  a  void  in  the 
concrete,  and  they  do  not  enter  into  the  calculated  strength  of 
the  floor.  The  calculations  require  considerable  time  if  exact, 
and  tables  VII  and  VIII  have  therefore  been  prepared  for  C  =  700 
and  5  =  20,000  and  16,000  respectively.  These  tables  show  at 
a  glance  the  depth  of  tile  and  thickness  of  concrete  required  for 
any  given  bending  moment,  together  with  the  corresponding 
pull  in  the  steel.  Note,  however,  that  the  bending  moment  must 
be  calculated  for  a  width  b  of  slab  equal  to  the  distance  between 
centers  of  ribs.  If  other  allowable  stresses  are  assumed  than 
those  for  which  the  tables  have  been  prepared,  we  may  easily 
prepare  new  tables.  We  have  in  all  the  preceding  formulas, 
that  the  bending  moment  is  directly  proportional  to  the  square 
of  the  coefficient  ci,  while  the  total  pull  in  the  steel  is  directly 
proportional  to  the  coefficient  c2.  But  we  have 

-  =  a  coefficient  times  c3 

A  glance  at  Table  IV  shows  that  c3  itself  is  practically  a  constant 
within  fairly  wide  limits,  so  that,  for  the  allowable  stresses  in 
ordinary  use,  we  may  make 

—  =  constant. 
c2 

It  follows  that  the  new  tables  are  prepared  from  the  tables  here 
given  by  multiplying  both  the  bending  moment  and  the  pull  in 
the  steel  of  the  old  table  with  a  factor;  this  factor  is  the  same 
for  both  items  and  is 

the  new  value  of  c2 

the  old  value  of  c2 

A  completed  floor  of  this  kind  is  shown  in  Figure  89. 

38.    Flat  Slabs.     If,  in  formulas  Sa  and  9a,  we  make  B  =  l, 
we  have  the  slab  formulas 

d  =  ^?i  IM     and  st 


But  the  load  on  the  slab  is  usually  given  and  in  Ibs./sq.  foot; 
denoting  by  w  the  total  dead  and  live  load  in  Ibs./sq.  foot,  the 
bending  moment  per  foot  width  becomes 

M  =  -  •     -      •  I2  -  12  tons-inches 


78 


REINFORCED   CONCRETE  BUILDINGS 


TABLE  VI.     0 
INCREASING  THE  DEPTH  FROM  d  TO  D  =  ad 


See  Table  IX  for  special  case .-  =  * 

o 


a  =  1.0 

1.0 

1.0 

1.0 

1.0 

1.0 

1.0 

1.0 

1.0 

1.0 

1.1 

.64 

.62 

.59 

.56 

.50 

.44 

.38 

.25 

.08 

1.2 

.54 

.50 

.46 

.41 

.34 

.26 

.15 

— 

— 

1.3 

.47 

.42 

.37 

.31 

.23 

.12 

— 

— 

— 

1.4 

.42 

.37 

.30 

.23 

.14 

.01 

— 

— 

— 

1.5 

.38 

.32 

.25 

.16 

— 

— 

— 

— 

— 

1.6 

.35 

.28 

.21 

.11 

— 

— 

— 

— 

— 

1.7 

.32 

.25 

.16 

.06 

— 

— 

— 

'— 

— 

1.8 

.30 

.22 

.13 

.01 

— 

— 

— 

— 

— 

1.9 

.28 

.20 

.09 

— 

— 

— 

— 

— 

— 

2.0 

.27 

.18 

.06 

— 

— 

— 

— 

— 

— 

n/b  = 

0.0 

0.1 

0.2 

0.3 

0.4 

0.5 

0.6 

0.7 

0.8 

TABLES  OF      = 

St  TONS 

TABLE   VII         S  =  20,000 


15 


700 


T  "  =7.0 







— 

— 

— 

393/20.6 

6.5 

— 

— 

— 

— 

— 

— 

372/20.0 

6.0 

— 

— 

— 

— 

— 

— 

350/19.3 

5.5 

— 

— 

— 

— 

— 

276/17.1 

329/18.6 

5.0 

— 

— 

— 

— 

204/14.9 

255/16.5 

308/17.8 

4.5 

— 

— 

— 

— 

189/14.3 

236/15.7 

284/16.9 

4.0 

— 

62/8.3 

93/10.1 

132/12.1 

174/13.6 

216/14.8 

259/15.9 

3.5 

— 

55/7.7 

85/  9.6 

120/11.4 

158/12.7 

196/13.8 

235/14.8 

3.0 

26/5.3 

48/7.2 

76/  9.1 

108/10.6 

141/11.7 

175/12.2 

210/13.6 

2.5 

21/4.8 

42/6.8 

67/  8.4 

94/  9.7 

123/10.6 

153/11.5 

184/12.3 

2.0 

17/4.4 

37/6.2 

57  /  7.6 

80/  8.6 

104/  9.4 

130/10.2 

158/10.9 

1.5 

14/3.9 

29/5.4 

46/  6.5 

65/  7.3 

85/  8.0 

107/  8.7 

132/  9.3 

1.0 

10/3.2 

21/4.4 

35/  5.0 

50/  5.7 

66/  6.4 

84/  6.9 

105/  7.5 

0.5 

6/2.3 

13/3.0 

23/  3.5 

33/  4.0 

46/  4.5 

60/  5.0 

76/  5.5 

0.0 

1/0.6 

4/1.1 

9/  1.6 

15/  2.0 

23/  2.5 

33/  3.0 

45/  3.5 

H  "  = 

4 

6 

8 

10 

12 

14 

16 

BENDING 


79 


q      2000 
TABLE   VIII         S  =  16,000 


C  =  700 


T  "  =  7.0 

_ 









— 

443/23.9 

6.5 

— 

— 

— 

— 

— 

— 

420/23.0 

6.0 



— 

— 

— 

— 

— 

396/22.2 

5.5 

— 

— 

— 

— 

— 

312/19.7 

372/21.4 

5.0 



— 

— 

— 

231/17.1 

288/19.0 

348/20.5 

4.5 



— 

— 

— 

214/16.4 

266/18.0 

320/19.4 

4.0 



70/9.5 

105/11.6 

149/13.9 

197/15.6 

244/17.0 

292/18.3 

3.5 



62/8.9 

98/11.0 

136/13.1 

178/14.6 

221/15.9 

266/17.0 

3.0 

29/6.1 

55/8.3 

86/10.5 

122/12.2 

159/13.5 

198/14.6 

238/15.7 

2.5 

24/5.5 

47/7.8 

75/  9.7 

106/11.1 

138/12.2 

173/13.2 

208/14.2 

2.0 

19/5.1 

41/7.1 

64/  8.7 

90/  9.9 

118/10.8 

147/11.7 

179/12.5 

1.5 

16/4.5 

33/6.2 

52/  7.5 

73/  8.4 

98/  9.2 

121/10.0 

149/10.7 

1.0 

11/3.7 

24/5.1 

40/  5.8 

56/  6.6 

74/  7.4 

94/  7.9 

119/  8.6 

0.5 

7/2.6 

15/3.5 

26/  4.0 

38/  4.6 

52/  5.2 

68/  5.8 

86/  6.3 

0.0 

1/0.7 

5/1.3 

10/  1.8 

17/  2.3 

26/  2.9 

38/  3.5 

51/  4.0 

H"  = 

4 

6 

8 

10 

12 

14 

16 

which  gives  the  very  convenient  formulas  (16) 

d  =-.J™  inches 
Ci    V   q 

and  st  =  c%d   tons.  (17) 

Here  the  span  I  is  expressed  in  feet,  and  the  factor  q  is  equal  to 
8  for  non-continuous  construction,  and  from  10  to  16  for  con- 
tinuous construction. 

39.  If  reinforced  in  both  directions,  and  supported  on  all 
four  sides,  the  slab  is  calculated  by  the  formulas  above,  dividing 
the  total  load  w  into  two  portions  w\  and  w2  where  wi  -f-  w2  =  w. 
If  we  denote  by  LI  and  L2  the  span  in  each  direction,  we  have  the 
arbitrary  formulas  for  the  division  of  the  load: 


and 


=  W 


w<>  =  w 


Li4  +  I*4 


The  heaviest  of  these  is  now  assigned  to  the  shortest  span,  and 
determines  the  depth  and  the  reinforcement  running  the  short 
way,  the  cross  reinforcement  is  designed  in  a  similar  manner 
using  the  other  load.  In  case  of  square  panels  the  two  loads 
become  equal,  each  one-half  of  the  total. 

The  formula  is  entirely  irrational  and  the  only  reason  it  is 


80 


REINFORCED  CONCRETE  BUILDINGS 


TABLE   IX    -  =| 

o 

/S  =  a  -  \/M«2-  1) 


a2 

a 

jS 

/3/a 

1.00 

.000 

1.000 

1.00 

1.01 

.005 

0.890 

0.89 

1.02 

.099 

0.847 

0.84 

.03 

.015 

0.815 

0.80 

.04 

.020 

0.789 

0.77 

.05 

.025 

0.767 

0.75 

.06 

.030 

0.747 

0.73 

.08 

1.039 

0.712 

0.69 

1.10 

1.049 

0.684 

0.65 

1.12 

1.058 

0.658 

0.62 

1.14 

1.068 

0.636 

0.60 

1.16 

1.077 

0.616 

'0.57 

1.18 

1.086 

0.596 

0.55 

1.20 

1.096 

0.579 

0.53 

1.25 

1.118 

0.541 

0.49 

.30 

1.140 

0.508 

0.44 

.35 

1.162 

0.479 

0.41 

.40 

1.183 

0.452 

0.38 

.45 

1.204 

0.430 

0.36 

.50 

1.225 

0.409 

0.33 

.60 

1.265 

0.371 

0.29 

.70 

1.304 

0.338 

0.26 

.80 

1.342 

0.308 

0.23 

.90 

1.378 

0.283 

0.21 

2.00 

1.414 

0.258 

0.18 

2.25 

1.500 

0.209 

0.14 

2.50 

1.581 

0.167 

0.11 

2.75 

1.658 

0.130 

0.078 

3.00 

1.732 

0.099 

0.058 

3.50 

1.870 

0.045 

0.024 

4.00 

2.000 

0.000 

0.000 

a2 

a 

0 

/3/a 

given  here  is  that  it  is  on  the  safe  side  and  better  than  other 
existing  formulas. 

The  supporting  girders  are  designed  with  reference  to  the 
load  brought  upon  them  by  the  particular  direction  which  they 
support,  and  the  bending  moment  is  often  increased  above  the 


BENDING  81 

calculated  because  the  load  seems  rather  more  concentrated 
towards  the  center. 


FIGURE    89.      TILE    CONCRETE    CONSTRUCTION,   READY  FOR 
PLASTERER 

Wise  Building,  Cleveland,  Ohio.     Alexis  Saurbrey,  Consulting 
Engineer 

Discussion  of  Tables  I  to  IX. 

1 


40.   TABLE  I.        x  = 


-3 

rC 

The  value  of  x  determines  the  location  of  the  neutral  axis, 
xd  being  the  distance  from  compression  face  to  neutral  axis. 


82  REINFORCED  CONCRETE  BUILDINGS 

We  have  seen  that  the  position  of  the  neutral  axis  within  the 
section  of  a  T-shaped  beam  leads  to  the  division  of  T-beams  into 
two  groups,  according  to  whether  the  neutral  axis  falls,  above 
or  below  the  bottom  line  of  the  flange.  In  this  latter  case  we 
introduce  the  coefficients  a  and  /?,  and  the  problems  contain  an 
arbitrary  element  which  is  absent  in  beams  of  the  first  type, 
where  the  dimensions  depend  mutually  upon  one  another  as  in 
formula  (8).  The  table  shows  that  the  neutral  axis  only  in 
exceptional  cases  approaches  the  middle  of  the  beam  where  it  is 
located  in  all  symmetrical  beams  following  Hooke's  Law  (steel 
for  example).  It  is  obvious  that  a  greater  amount  of  steel  is 
required  for  low  steel  stresses  than  for  high;  we  therefore  see 
that  the  neutral  axis  is  lowered  by  increasing  the  amount  of  steel. 


41.   TABLE  II.        Cl  =  \J\Cx  (\  -  ±x 

,       12.9       I'M 
We  have         d  =  - 


The  smallest  possible  value  of  d  is  obtained  when  a  large  value 
of  Ci  is  used,  or,  in  other  words,  when  high  concrete  stresses  are 
combined  with  low  steel  stresses.  The  influence  of  the  concrete 
stresses  is  much  more  pronounced  than  that  of  the  steel  stresses; 
it  is,  therefore,  not  economy  to  increase  the  amount  of  steel  in 
order  to  save  on  the  concrete.  It  is  not  impossible  to  analyze 
this  problem  mathematically,  but  owing  to  variation  in  unit 
prices  it  seems  hardly  worth  while.  The  possibility  of  decreas- 
ing the  depth  of  construction  by  using  high  concrete  stresses 
and  low  steel  stresses  may,  however,  be  of  importance  in  special 
cases  where  the  head  room  is  limited. 

Cx 
42.  TABLE  III.        C2  =  333 

The  total  pull  in  the  steel  is          st  =  02.  •  ^  •  d. 


The  total  amount  of  steel  is          a  =  st 


12 

2000 
S 


2000          bd  fa\fS 

so  that  -£-  '  C2 '  12     °r     C2  =  (bd)  VT66 

The  coefficient  c2,  then,  is  a  measure  of  the  amount  of  steel  used 


BENDING  83 

for  a  given  cross-section,  bd  being  the  area  of  the  cross-section 
in  square  inches.     We  note  that 

^-,  times  100 
bd 

is  the  percentage  of  steel  required  for  the  beam;  if  we  denote 
the  percentage  by  p  we  have 

16600 


This  expression  has  been  used  in  calculating  Table  V. 

43.  TABLE  IV.     c3  =  1  -  I  x 

In  general  the  total  pull  in  the  steel  is  obtained  by  dividing 
the  bending  moment  by  a  certain  lever  arm  di,  equal  in  length 
to  the  distance  between  the  centers  of  compression  and  tension. 
Reference  to  Figure  84  gives  at  once 

di  =  |  xd  +  (1  -  x)  d  =  (1  -  \x)  d  =  c,d 

When  d  is  known  we  get  d\  as  c$d;  Table  IV  gives  the  values 
of  c3  for  various  combinations  of  stresses. 

44.  TABLE  V.     p  =  ^  X  100  =  c2 

The  percentage  of  steel  has  but  little  interest  for  the  prac- 
tical designer  as  the  problems  usually  present  themselves. 
The  table  is  added  for  the  convenience  of  those  who  are  in  the 
habit  of  selecting  the  percentage  of  steel  rather  than  determine 
the  allowable  stresses.  The  table  is  correct  only  for  such  beams 
where  a  =  1  and  r  =  15. 

45.  TABLE  VI  will  be  found  useful  when  designing  T-beams 
of  larger  than  minimum  depth.     When  we  have  selected  «,  as 
explained  in  connection  with  formula  12,  etc.,  the  correspond- 
ing ft  is  found  by  Table  VI  for  any  value  of  n/b.     The  method 
of  design  will  be  clearly  evident  from  the  example  in  Article  47. 
Table  IX  is  a  more  extensive  table  for  the  special  case  where 
n/b  =  J,  which  is  a  common  value  in  practice.     The  variation 
of  n/b  does  not  affect  the  values  very  much,  so  that  for  small 
values  of  a  Table  IX  may  be  used  for  other  values  of  n/b  than  J. 

46.  TABLES  VII  and  VIII.     Tile-concrete  floors. 

The  use  of  these  tables  is  best  explained  by  an  example. 
The  span  of  the  flat  portion  is  20  feet;  the  total  dead  and  live 


84  REINFORCED  CONCRETE  BUILDINGS 

load  is  assumed  to  be  250  Ibs.  per  square  foot.     With  4"  ribs 
we  get  the  width  of  beam 

b  =  4  +  12  =  16" 
and  the  corresponding  bending  moment  in  inch-tons 

250 
M  =  *  x  ^  x  2°2  X  16  =  100  inch-tons. 


If  the  allowable  stresses  are  S  =  20,000  and  C  =  700,  we  must 
use  Table  VII,  and  we  see  at  once  that  we  can  use  either  a  10  " 
tile  with  about  2f"  concrete,  or  a  12"  tile  with  2"  concrete. 
As  we  do  not  wish  to  have  less  than  2"  of  concrete  over  the  tiles, 
we  cannot  use  the  larger  tiles  economically.  If  we  select  12" 
tiles  and  2"  concrete,  the  corresponding  pull  in  the  steel  is  9.4 
tons  according  to  the  table,  requiring  .94  square  inches  of  steel, 
for  instance,  one  f  "  square  and  one  f  "  square  bar. 

It  should  not  cause  surprise  that  the  moments  tabulated  in 
Table  VIII  are  larger  than  the  corresponding  values  of  Table 
VII,  although  the  allowable  stress  on  the  steel  is  smallest  in 
Table  VIII.  The  explanation  is  given  in  the  remarks  under 
Table  II,  Article  41,  and,  in  accordance  with  the  statements 
made  there,  it  will  be  seen  that  the  larger  moments  of  Table 
VIII  are  obtained  only  by  increasing  the  steel  areas. 

TABLE  IX.     See  Table  VI,  Article  45. 

47.  EXAMPLE  1.  T-Sections.  Continuing  the  example  given 
above  under  the  discussion  of  Tables  VII  and  VIII,  we  proceed 
as  follows  to  design  the  girder: 

The  load  on  the  floor  is  250  Ibs.  per  square  foot,  the  span  of 
the  flat  portion  on  each  side  of  the  girder  is  20'  —  0",  and  the 
girder  therefore  carries  a  load  of  250  X  20  =  5,000  Ibs.  per 
lineal  foot,  to  which  should  be  added  the  weight  of  the  girder 
itself.  Assuming  this  item  to  be  included  in  the  5,000  Ibs.,  and 
assuming  a  span  of  24'  —  0"  for  the  girder,  the  bending  moment 
on  the  girder  becomes 

M  =  J  X  x  242  X  12  =  2160  inch-tons. 


We  decide  to  use  high  tension  steel,  for  which  S  =  20,000,  and 
we  allow  C  =  700  Ibs.  per  square  inch  on  the  concrete.  We  get 
then  from  Table  II  :  Ci  =  10.3;  from  Table  III  :  c2  =  .72;  and  from 
Table  I:  x  =  .344,  and  we  may  now  proceed  with  the  design, 
using  formulas  (8a),  (9a),  (lOa),  and  (11).  The  width  of  flange, 


BENDING  85 

B,   may   be  selected   arbitrarily.     Let  us  make   B  =  4'  —  0". 
Then,  by 


(9a)    .........  s  =  c2BD  =  .72  X  4  X  29.1  =  84  tons. 

(11)    .........  a  =  ?°  -  st  =  X  84  =  8.4  square  inches. 


(10a)    t  =  xd  =  .344  X  29.1  =  10" 

We  have  to  make  the  stem  of  the  beam  wide  enough  to  accom- 
modate 8.4  square  inches  of  steel,  say  n  =  12",  and  the  girder 
is  then  designed  as  far  as  concerns  the  bending  moment.  Ques- 
tions pertaining  to  shear,  etc.,  will  be  considered  later. 

We  can,  if  we  desire,  reduce  the  thickness  t  of  the  flange  by 
increasing  the  depth  d.  While  this  operation  is  not  always  neces- 
sary, or  even  desirable,  we  will  nevertheless  continue  the  ex- 
ample to  show  the  method  of  procedure. 

If,  then,  we  increase  the  depth  from  29.1"  to,  say,  35",  we  get 

a  =  J^_  =  a6a>2 

the  coefficient  a  indicating  the  proportionate  increase  in  the 
depth.  The  value  of  ft  is  next  obtained  by  Table  IX,  remem- 
bering that 

™  —  i  _     OK 
6  "  *~ 

the  stem  being  12"  wide  and  the  flange  48". 

For  -  =  .25    and    a  =  1.2,   Table   IX  gives  ft  =  .43;   then, 
6 

by  the  theory  outlined  for  this  case,  Article  36,  we  have 
The  new  depth  D  =  ad  =  1.2  X  29.1  =  35". 

s         84 
The  new  pull  in  steel  s&  =  —  =  y-x  =  70  tons. 

The  new  thickness  of  flange       T  =  ftt  =  .43  X  10  =  4.3". 

It  is  of  course  unnecessary  to  calculate  the  dimensions  of 
the  "  minimum  "  beam  first,  as  done  here,  unless  we  expressly 
desire  to  have  these  dimensions.  Let  us,  for  instance,  again 
consider  a  given  bending  moment  of  2,160  inch-tons;  let  us 


86 


REINFORCED  CONCRETE  BUILDINGS 


further  select  arbitrarily  the  width   B  =  4'  —  0",  and    let  us 
finally  choose  the  coefficient  a  =  1.2,  then,  by  Table   IX,  we 

77 

get  ft  =  .43  for  an  estimated  value  of  —  =  .25;   and  we   also 

o 


have  a2  =  1.44.     We  may  now  find  the  dimensions  directly,  by 

12.9      /M  12.9  2l60 

'^'V£-  = 


12.9 
<X 


(14) 


8t  =  -*  .  B  -  D  = 


1.44 


03 
X  4  X  35 


_ 


70  tons. 


........  T==a":r'/)==rx  -344  x  35  =  4-3" 

Figure  90  shows  the  resulting  construction.     The  space  Z 


FIGURE  90. 

under  the  flange  may  be  taken  up  of  tiles  with  less  depth  than 
those  used  for  the  balance  of  the  floor,  as  long  as  the  thickness 
of  concrete  over  these  lighter  tiles  is  made  equal  to  the  thick- 
ness of  flange  just  found,  or  thicker.  In  this  way,  the  construc- 
tion of  the  girder  is  lightened  somewhat,  and  the  form  work 
may  be  made  of  the  same  light  construction  up  to  the  face  of 
the  stem.  A  floor  constructed  in  this  manner  also  presents  a 
tile  surface  to  receive  the  plaster  in  all  places  except  on  the 
stem  of  the  girder. 

If  we  now  wish  to  check  our  calculations,  we  may  proceed 
as  follows: 

From  Table  IV,  we  get  at  once  c3  =  .89,  and  the  lever  arm 
of  stresses  becomes 

D!  =  c3D  =  .89  X  35  =  31.2" 
and  we  get  the  total  pull  in  the  steel 

M       2160 
S<  =  Si  =  3L2  =  69'3 


BENDING  87 

The  calculations  above  gave  70  tons.  We  now  have  to  find  the 
compressive  resistance  of  the  beam,  and  this  we  get  as  the  dif- 
ference between  two  items :  A  =  the  total  resistance  of  the  entire 
area  above  the  neutral  axis,  and  B  =  the  section  cut  out  be- 
tween the  neutral  axis  and  the  bottom  of  the  flange.  First  we 
find  the  location  of  the  neutral  axis 

xD  =  .344  X  35  =  12.25" 
and  we  get  then 

A  =  \  X  48"  X  12.25"  X  700  =  205,800  Ibs. 
To  find  B,  we  must  first  find  the  concrete  stress  at  the  bottom 
of  the  flange,  and  by  reference  to  Figure  86,  we  get 

CT  =  700  x  12^2  ^54'3  =  70° x  i^nl  =  454  lbs-  per  s<i- inch- 

When  determining  B,  we  must  remember  that  the  "  width  of 
beam  "  is  not  48",  but  48"  -  12"  =  36",  the  12"  being  the 
width  of  the  stem,  and  we  get  now 

B  =  \  X  36  X  7.95  X  454  =  65,000  lbs. 
Then 

A  -  B  =  205,800  -  65,000  -  140,800  lbs.  =  70.4  tons. 
The  total  compression  must  of  course  equal  the  total  resist- 
ance, and  we  see  that  our  design  is  correct  as  this  is  the  case. 
The  slight  difference  between  the  70  tons  of  the  design  and 
the  70.4  tons  of  the  check  is  due  to  the  inaccuracies  of  the  slide 
rule  and  the  various  interpolations  made,  and  is  entirely  too 
small  to  warrant  further  investigation. 

48.   EXAMPLE  2.  —  FLAT  SLABS. 

Given:  Live  plus  dead  load  500  lbs.  per  square  foot.  Span 
12'  —  0"  between  centers  of  support.  Allowable  stresses, 
concrete,  600  lbs.,  steel,  14,000  lbs.  Continuous  construction 
U=10.  -  ). 

We  get  from  Table  II:  ci  =  10.1;  Table  III:  c2  =  .71;  and 
we  can  now  proceed  with  the  design  using  formulas  (16)  and 
(17),  and  we  get 

I       /w       12.0      /500 
~c1'\-q==Wl'\^'- 

and     st  =  c2d  =  .71  X  8.4  =  5.96  tons  per  lin.  ft.  of  width. 

To  the  depth  must  now  be  added  the  amount  of  concrete  required 
to  properly  protect  the  rods. 


88 


REINFORCED  CONCRETE  BUILDINGS 


49.  To  Find  the  Stresses  in  a  Given  Beam,  when  the  bending 
moment  is  known,  requires  a  knowledge  of  the  ratio  S/C  and  of 
the  ratio  r.  A  simple  mathematical  analysis  of  the  beam  gives 
the  first  ratio,  as  we  shall  see  presently;  the  value  of  r  we  cannot 
determine,  so  that  it  will  have  to  be  assumed  in  the  same  manner 
as  when  designing.  The  value  15  may  very  well  be  used. 

We  found  above  the  expression  (Article  35,  Formula  12) 


-n/b 

while  Article  36  gives  the  means  for  eliminating  in  this  expres- 
sion first  (3  and  a;  by  means  of  (6)  and  (11),  c2  and  st  are  elim- 
inated. The  resulting  equation  may  be  solved  for  S/C,  and 
gives : 


S 


T'H'ii-'^}- 


(18) 


The  quantities  entering  on  the  right  side  of  the  equation  mark 
are  all  known  except  r;  in  Figure  91  the  several  dimensions  are 


FIGURE  91. 

shown;  the  value  V  is  written  as  an  abbreviation  of  ra/6,  so 
that 


represents  the  thickness  of  an  imaginary  strip  of  concrete  hav- 
ing the  same  width  as  the  beam  considered,  and  the  same  resist- 
ance as  the  tension  steel;  a  and  b  are  taken  directly  from  the 
drawing,  same  as  the  other  dimensions. 

In  special  cases  this  formula  is  greatly  simplified,  although 
there  is  no  difficulty  whatever  in  using  the  formula  given. 


BENDING  89 

(1)  For  rectangular  beams,  or  slabs,  we  have  n  =  b  and 
T  =  0  and  we  get 

J'-i  +  J^1?  M 

(2)  Disregarding   the   influence   of   the   stem   on  the   com- 
pression, as  is  sometimes  done,  we  have  n  =  0  and  get 

S  =      2D  -  T 

C~  ?T\       2  Da  (20) 

\rj        Tb 

(3)    If,  in  Formula  18,  T2  =  2V H,  we  find  after  some  re- 
duction 

S_  =  Tb 
C  ~  2a 

The-  value  of  n/b  disappears  entirely,  which  evidently  means 
that  the  neutral  axis  coincides  with  the  bottom  of  the  flange. 

The  ratio  —^-rr    is   therefore    the  criterion  of  the  section.     If 
2  V  H 

=  1,  the  neutral  axis  coincides  with  the  bottom  of  the  flange, 
if  <  1,  it  falls  below,  if  >  1,  above  the  bottom  of  the  flange. 

If  we  now  wish  to  determine  the  stresses  in  a  given  beam,  we 
begin  by  selecting  r,  next  we  determine  the  value  of  the  cri- 
terion, so  that,  if  equal  to  unit,  we  use  formula  (21),  while  if 
larger  than  unit,  we  use  formula  (19),  and  if  smaller,  the  orig- 
inal formula.  Then  the  location  of  the  neutral  axis  is  calcu- 
lated from 

1 


and  the  coefficient  c3  =  1  —  f  x  is  determined  from  the  value 
of  x  just  found.     The  effective  depth  is  then 

where  d  is  the  depth  from  ultimate  compression  fiber  to  the  cen- 
ter of  the  steel.     We  have  then 

s   =- 

and,  by  (11) 

2000 


90 


REINFORCED  CONCRETE  BUILDINGS 


We  know  now  S  and  S/C,  and  it  is  a  simple  matter  to  determine 
C. 

50.    EXAMPLE  3.     T-SECTION. 

Given  the  beam  shown  in  Figure  92,  find  the  stresses,  when 
the  bending  moment  is  2,160  inch-tons.  We  have 


T2 


4.5  X  4.5 


= 20.25 

2  VH  ~  2  X  2.19  X  30.5  "  113.5 


V7  sq.  inches 

FIGURE  92. 


which  is  -evidently  <  1.     Using  therefore  formula  (18)  we  find 

325.4 


_  15X 
C  ~     5  X  168.4 


Now 


x  = 


1 


^  =  .342 


and  c3  =  1  -  J  a;  =  0.886. 

Hence  di  =  c3d  =  0.886  X  35  =  31" 

Bending  moment  2,160  inch-tons,  then 
2160 


31 


=  69.6  tons. 


69.6  =  20,100 


„       20100       „„, 

c  =  W  =  695' 


or  about  20,000  and  700  Ibs. /square  inch  for  steel  and  concrete, 
respectively. 

51.   EXAMPLE  4.     T-SECTION.     Special  case. 

Given  the  beam  shown  in  Figure  93. 

We  have  V  =  ™  =  15  X  ~  =  2.62. 


10  X  10 


=  100 
2  VH  ~  2  X  2.62  X  19.1  ==  100 


=  1. 


BENDING 


91 


Use  formula  (21)  which  gives 

S       10  X48 
C  = 


=  28.6 


2  X8.4 

The  balance  of  the  calculations  may  now  be  continued  ex- 
actly as  in  the  preceding  example. 


j-8.4  sq.  inches 

FIGURE  93. 

52.    EXAMPLE  5.     RECTANGULAR  BEAM.     Slabs. 
Given  the  rectangular  beam  shown  in  Figure  94;  we  use 
formula  (19)  which  gives 


C  2  +2\ 


V_0.85  sq.  inches 

FIGURE  94. 
225  +  ^  ' 


0.85 


CHAPTER   VII 

TRANSVERSE  STRESSES.  —  U-BARS 

53.  IN  addition  to   the   longitudinal   stresses   examined   in 
the   preceding   articles,   transverse   stresses   exist   in   reinforced 
concrete  beams  as  well  as  in  beams  of  other  materials.     But 
the  transverse  stresses  are  different  in  trusses  and  in  solid  beams : 
in  the  truss,  each  individual  member  is  stressed  in  its  longi- 
tudinal direction  only,  and  there  is  no  shear.    In  the  solid  beam, 
longitudinal   stresses   exist   in   the   top   and   bottom   chords   or 
fibers,  and  the  web  is  then  subject  to  shear  stresses  both  longi- 
tudinally and  transversely.     In  special  cases  these  shear  stresses 
may  vanish,  as  for  instance  in  the  I-shaped  steel  beam  of  vari- 
able depth,  when  the  ratio 

bending  moment  at  any  point 
depth  at  the  same  point 

is  a  constant.     This  is  the  case  in  a  parabolic  girder  loaded 
over  its  entire  length  with  a  uniformly  distributed  load. 

54.  In  view  of  this  difference  between  trussed  beams  and 
solid  beams,  it  becomes  necessary  to  decide  whether  to  treat 
the  reinforced  concrete  beam  as  the  one  or  the  other.     To  the 
eye  a  reinforced  concrete  beam  certainly  appears  solid  enough, 
and  such  is  indeed  the  case  when  the  beam  is  first  made  and  the 
load  is  being  put  on.     But  when  the  load  reaches  a  certain  in- 
tensity the  " solidity"  of  the  beam  is  destroyed.     Slight  cracks 
soon  become  evident,  at  least  when  arrangement  has  been  made 
to  observe  them,  and  that  under  loads  corresponding  to  a  steel 
stress  of  from  4,000  to  6,000  Ibs./square  inch,  or  a  concrete 
stress  of  350  Ibs./square  inch.     It  follows  that  under  the  ordi- 
nary working  load  our  reinforced  concrete  beam  is  perforated 
with  cracks  extending  from  the  bottom  fiber  up  toward  the 
neutral  axis,  without  quite  reaching  the  neutral  axis,  so  that, 
under  any  circumstances,  the  beam  is  certainly  not  a  "solid" 
beam.     These  hair  cracks  have  been  noted  by  all  who  have 

92 


TRANSVERSE  STRESSES.  — U-BARS  93 

taken  the  trouble  to  look  for  them  with  but  one  exception 
(Considere) ;  they  are  not  an  occasional  occurrence,  but  a  uni- 
versally recognized  phenomenon  of  the  greatest  importance  for 
our  understanding  of  the  stresses  within  a  reinforced  concrete 
beam.  The  presence  of  these  cracks  is  accounted  for  by  the 
simple  fact  that  concrete  is  unable  to  stretch  as  much  as  steel 
before  cracking,  so  that,  under  a  certain  load,  the  concrete  refuses 
to  follow  the  steel  in  its  elongation  and  goes  to  pieces.  The 
cracks  of  this  class  appear  throughout  the  length  of  the  beam, 
fairly  uniformly  spaced,  and  increase  in  size  with  increasing 
load. 

55.  The  crack  of  course  is  an  open  space  existing  between 
surfaces  which  at  some  earlier  time  were  in  close  contact  and 
united.     We  must  now  understand  as  a  fundamental  principle 
that  stresses  cannot  be  transmitted  through  open  cracks.     Com- 
pression may  be  transmitted  through  a  contact  only,  and  fric- 
tion may  exist  on  surfaces  pressed  together,  but  no  kind  of 
stress  will  jump  across  an  open  space.     It  follows  that  shear 
in  the  ordinary  sense  of  the  word  cannot  exist  in  a  reinforced 
concrete  beam  loaded  above  a  certain  limit,  because  the  nature 
of  shear  requires  equal  intensity  on  a  horizontal  and  a  vertical 
plane,  and  this  is  of  course  impossible  when  the  beam  has  ver- 
tical cracks.     Or,  we  may  simply  say  that  the  vertical  shear 
cannot  exist  in  the  crack  itself.     Where  a  crack  occurs  there  is 
therefore  nothing  but  the  compression  flange  and  the  tension 
steel  to  carry  the  shear,  —  a  distribution  of  the  shear  which  is, 
to  say  the  least,  not  easily  reconciled  with  current  ideas  of  shear 
in  solid  beams. 

56.  Entirely  different  from  these  hair  cracks  are  the  much 
larger,  pronounced  failure  cracks  which  predict  the  approach- 


.1  t 

FIGURE  95. 

ing  collapse  of  the  test  beam.  If  located  at  or  near  the  point 
of  maximum  bending  moment,  they  are  undoubtedly  due  to 
excessive  elongation  of  the  steel  disclosing  a  failure  by  tension 
in  the  steel;  if  near  the  end,  the  crack  usually  takes  the  shape 
shown  in  Figure  95,  either  with  or  without  the  horizontal  crack 


94  REINFORCED  CONCRETE  BUILDINGS 

D.  The  vertical  crack  E  is  wide  open,  especially  at  the  bottom, 
decreasing  in  width  as  it  approaches  the  top  of  the  beam.  As 
the  steel  stress  at  this  point  certainly  cannot  exceed  the  steel 
stress  at  the  point  of  maximum  bending  moment,  this  crack  is 
not  due  to  excessive  tensile  stresses  in  the  steel.  It  must  be 
due  to  sliding  of  the  reinforcement:  the  steel  is  pulling  out  of 
the  end  of  the  beam  at  the  same  time  bursting  its  concrete 
envelope,  and  causing  the  horizontal  crack.  Let  it  be  under- 
stood that  no  amount  of  shear  will  cause  a  gaping  crack,  but  once 
sliding  sets  in  and  causes  the  vertical  crack,  it  is  clear  that  the 
one  end  of  the  beam  will  be  compelled  to  revolve  around  the 
other  end,  causing  in  the  first  place  the  double-curved  line 
of  cleavage,  and,  secondly,  great  friction  on  the  surfaces  of 
contact. 

57.  The  above  remarks  lead  to  the  conclusion  that  a  concrete 
beam  is  a  solid  beam  up  to  a  certain  load  at  which  point  the 
tensile  resistance  of  the  concrete  is  exhausted,  and  a  readjust- 


FIGURE  96. 


ment  of  stresses  takes  place  within  the  beam.  This  readjustment 
is  different  for  different  types  of  beams.'  In  a  rectangular 
beam  (Figure  96)  we  may  well  assume  that  the  com- 
pression follows  lines  as  AC  and  BC  when  the  load  is  placed 


FIGURE  97. 


at  C;  if  the  load  moves  to  D,  the  lines  change  to  AD  and  DB. 
Under  these  circumstances  there  is  no  shear,  at  least  not  in  the 
ordinary  sense  of  the  word.  We  may  compare  a  system  of  this 


TRANSVERSE  STRESSES.  —  U-BARS 


95 


kind  to  a  triangular  frame  with  hinged  corners  (Figure  97). 
The  chords  AC  and  BC  will  be  in  compression,  and  the  chord 
A B  in  tension,  hence  at  A  and  B  the  hinges  are  subject  to  severe 
stresses.  The  same  is  the  case  at  A  and  B  in  the  reinforced 
concrete  beam,  so  that  the  "  length  of  embedment  "  AE  and  BF 
in  Figure  96  must  be  made  long  enough  to  prevent  sliding  of 
the  rod.  The  shear  existing  in  a  system  of  this  kind  is  that 
negligible  quantity  caused  by  the  stiffness  of  the  system  as  a 
whole,  a  kind  of  friction  caused  by  the  lack  of  flexibility  at  the 
supposed  hinges. 

The  system  A  BC  is  an  equilibrium  curve  for  the  load  C 
and  the  reactions  A  and  B',  this  same  argument  would  of  course 
hold  true  for  any  number  of  forces,  or  even  for  uniformly  dis- 
tributed loads,  in  which  latter  case  the  compression  curve  would 
be  a  continuously  arched  curve  from  A  to  B  (when  the  load 
covers  the  entire  span).  But  if  the  beam  under  consideration 
is  a  T-beam  instead  of  a  rectangular  beam  it  becomes  impos- 
sible to  make  the  compression  line  curve  down  to  the  support- 
ing points,  except  for  a  width  equal  to  that  of  the  stem.  A 
T-beam  (Figure  98a)  may  be  considered  as  consisting  of  two 


FIGURE  98a. 


FIGURE  986. 


FIGURE  98c. 


beams  side  by  side;  a  T-beam  proper  (Figure  986)  and  a  rect- 
angular beam  (Figure  98c).  In  this  rectangular  portion  it  is 
quite  possible  for  the  compression  lines  to  dip  down  at  the  sup- 
ports, but  not  so  for  the  T-beam  portion,  there  being  no  con- 
crete left  to  carry  the  stresses  down  to  the  steel.  This  leads 
to  the  idea  of  bending  the  steel  up  over  the  support  to  meet 
the  compression  flange,  reversing  the  conditions  shown  in 
Figure  96. 

58.  Let  us  consider  a  portion  of  a  reinforced  concrete  beam 
between  two  points  a  and  b  (Figure  99).  The  bending  moment 
at  a  is  denoted  by  Ma,  the  distance  between  center  of  com- 


96 


REINFORCED  CONCRETE  BUILDINGS 


pression  and  center  of  tension  by  da,  then  the  total  pull  in  the 
steel  at  a  is 

Ma 
Sa   =    -T- 


and  at  b 


sb  = 


Mi 

db 


FIGURE  99. 

The  difference  between  these  two  is 

_Ma_  Mb 
'Sa     Sb  -  Ta      ~db 

The  prefix  A  simply  denotes  the  difference  in  the  item  con- 
sidered so  that  As  means  the  variation  of  s  between  the  points 
in  question. 

It  is  now  evident  that  the  portion  abed  is  subject  to  two 
pulling  forces  acting  near  its  lower  end  cd:  one  force  sa  pulling 
toward  the  left,  another  pulling  toward  the  right,  sb.  If  sa  is 
larger  than  sb,  the  end  cd  must  have  a  tendency  to  move  toward 
the  left  in  precisely  the  same  manner  as  if  pulled  that  way  by 
a  force  equal  to  the  difference  of  the  two  pulling  forces;  we 
may  therefore  consider  the  force  As  =  sa  —  sb  as  acting  alone. 
This  condition  is  represented  in  Figure  100,  and  this  diagram 


5 

*f 

f 

F 

f            1 

f             ) 

h 

As^  

1 

I 

f 

B 

^j  ^   Al 

FIGURE  100. 

shows  at  once  that  cdef  is  a  cantilever  fixed  at  its  base  ef,  and 
loaded  near  its  end  with  a  load  As.  The  depth  ce  we  do  not 
know  at  the  present  time;  let  us  indicate  this  unknown  quan- 


TRANSVERSE  STRESSES.  —  U-BARS 


97 


tity  by  h.  The  bending  moment  on  the  cantilever  is  then  h  As; 
the  arm  of  "  the  couple  of  stresses  "  in  the  cantilever  is  c3  AZ; 
hence  if  a  vertical  reinforcing  rod  is  disposed  near  bd  the  pull  on 
this  rod  becomes 


But  this  pull  k  can  exist  only  when  counterbalanced  by  a  cor- 
responding compression,  so  that  the  beam  becomes  a  trussed 
beam  as  shown  in  Figure  101.  The  vertical  reinforcement 


J        /] 

xx              K              |> 

h       K 

XI       Fl 

X     X      1  ^      1 

I    N    N 

*     1  x    1^ 
\t     \|    \j 

At 

Ra 

B 

C!RC 

FIGURE  101. 

designed  in  this  manner  is  usually  made  of  a  bar  bent  to  U- 
shape  and  circling  the  main  tension  rod  (Figure  102a,  6);  they 
are  therefore  called  U-bars  or  stirrups.  The  U-bar  is  unneces- 


FIGURE 102a. 


FIGURE  1026. 


sary  when  k  =  0,  which  is  always  the  case  when  As  =  0:  i.e., 
when  either  the  tension  chord,  or  the  compression  chord,  or 
both  together,  follow  the  equilibrium  curve.  As  shown  above, 
this  is  always  the  case  in  a  rectangular  beam  with  well-anchored 
reinforcement,  and  it  is  also  the  case  for  such  parts  of  a  T-beam 
in  which  the  reinforcement  is  bent  up  to  follow  the  equilibrium 
curve.  In  all  other  cases  k  has  a  definite  value.  For  straight 
reinforcement  and  straight  top  chord,  we  have  (Figure  103) : 

da  =  db  =  Cj,d 

Ma-   Mb 


hence 


As  = 


98 


REINFORCED  CONCRETE  BUILDINGS 


where 

and 

hence 

or 


Ma  =  Ra  - 
Mb  =  R  (a  -  AQ  -  2P  (p  -  AZ) 
Ma  -  Mb  =  AZ  (R  - 


AS  =      -  (R  - 


and  k  =  —^  (R  - 

It  is  now  easy  to  understand  that  the  length  h  cannot  exceed 

1r. 


rsr 


a-Al 


FIGURE  103. 

the  distance  from  the  center  of  the  steel  to  the  neutral  axis. 
This  gives 

h  (1  -  x)d  1  -  x 


The  value  of  this  expression  cannot  exceed  unit;  for  ordinary 
cases  its  value  is  about  three-fourths.  Hence  the  maximum 
possible  value  of  k,  for  the  conditions  named,  is: 

kmax   ==   K  Zr.  (23) 

N 

A    , 


mi: 


FIGURE  104. 

59.  It  is  now  interesting  to  note  that  this  same  expression 
may  be  obtained  directly  in  the  simplest  manner.  Let,  in 
Figure  104,  the  section  A  B  remove  the  right  end  of  the  beam 


TRANSVERSE  STRESSES.  —  U-BARS  99 

leaving  the  main  tension  bar  and  the  U-bar  projecting.     The 
stress-resultants  acting  upon  A  B  are  then,  when  the  chords  are 
parallel:    the   horizontal    compression    X,    the    horizontal    pull 
Y'j  and  the  vertical  force  k  in  the  U-bar.     The  loads  are  PI,  P2, 
etc.,  and  the  reaction  R.     If  we  now  project  on  a  vertical  line 
MN,  the  horizontal  stresses  vanish  and  we  have 
R-  (P!  +  P2  +  P3  +  ......  )  =k 

or  k  =  R  -  2P. 

60.  The  beam  with  straight  top-  and  bottom-chords  is  an 
exception.  Usually  the  stem  of  a  T-beam  may  be  considered 
as  in  equilibrium,  and  in  addition  some  of  the  bars  are  bent 
up  in  the  T-beam  portion  to  approximate  the  equilibrium  curve, 
so  that  a  material  reduction  in  the  value  of  k  takes  place  in  all 
practical  beams.  With  the  notations  of  Figure  98  we  have, 
on  account  of  the  stem,  a  reduction  equal  to 

b  —  n     ,  j       b  —  n  ro       vrn 

—r—      hence     k  =  —  r  —  [R  —  2P] 

If  out  of  the  total  number  x  of  bars  in  the  T-beam,  a  certain 
number  y  follow  the  equilibrium  curve,  we  have  a  further  re- 
duction equal  to 

fc  =  ?LH»  .  *r_»  .   [R  _  2P]        (24) 


x  x  b 

Thus,  if  b  =  48"  and  n  =  12",  we  have 

b  -  n  _  48  -  12  _  3 
~b~  48        ~  4 

so  that,  out  of  a  total  number  of  say  eight  bars,  the  six  belong  to 
the  T-beam.  If  out  of  these  six,  two  are  bent  as  required,  we 
have  x  =  6  and  y  =  2,  hence 

k  =  i  X  i  X  [R  -  2P]  =  }  -  [R  -  SP] 

61.  Thus,  in  order  to  calculate  the  stress  on  the  U-bars, 
it  becomes  necessary  to  know  the  properties  of  the  curve  of  equi- 
librium for  the  system.  When  the  loads  are  stationary,  the 
curve  is  drawn  as  a  force  polygon  to  the  actual  loads  and  reac- 
tions. For  a  uniform  load,  covering  the  entire  span,  this  curve 
is  a  parabola;  it  is  not  practical  to  bend  the  bars  to  this  shape, 
but  it  may  be  closely  approximated  by  a  system  of  bars  with 
straight  portions  between  the  several  bents.  A  uniformly 


100  REINFORCED  CONCRETE  BUILDINGS 

distributed,  moving  load  has  no  definite  curve  of  equilibrium, 
so  that  in  that  case  the  most  dangerous  position  of  the  load 
must  be  found  and  the  U-bars  proportioned  according  to  For- 
mula 23  above,  while  the  bent  bars  are  arranged  to  meet  the 
requirements  of  some  particular  type  of  loading,  for  instance, 
the  total  load.  Similarly,  concentrated  loads  may  be  either 
stationary  or  moving.  In  buildings  the  concentrated  loads 
are  usually  stationary.  The  given  load  is  a  uniform  load,  so 
that  the  beams  are  loaded  as  explained  above;  these  beams 
in  turn  frame  into  the  girders,  one,  two,  or  three  beams  to  each 
span,  and  these  concentrated  beam  loads  are  stationary.  It 
is  a  simple  matter  to  bend  the  main  tension  bars  to  conform  to 
this  type  of  loading;  examples  are  given  in  Figures  105  and  106. 

I      I 


FIGURE  105.  FIGURE  106. 

The  moving  concentrated  load  is  usually  found  only  in  structures 
like  highway  bridges,  subject  to  steam-roller  traffic,  in  crane- 
track  girders,  etc.  In  such  cases,  the  live  load  is  large  in  pro- 
portion to  the  dead  weight  of  structure  and  covering,  so  that 
the  T-beams  are  usually  not  economical  structures  for  this  class 
of  girders.  They  may  be  constructed  by  using  the  adequate 
number  of  U-bars;  or  rectangular  beams  may  be  used  of  the 
required  cross-section. 

62.  The  problem  of  designing  a  T-beam  under  a  uniform 
load  confronts  the  reinforced  concrete  designer  every  day.  It 
is  customary  to  consider  the  load  as  covering  the  entire  span, 
except  in  cases  where  it  is  expressly  stipulated  that  the  most 
dangerous  position  of  the  load  shall  form  the  basis  for  the  cal- 
culation of  the  U-bars.  Arguments  may  be  advanced  pro  et 
con.,  —  usually  the  load  specified  is  a  maximum  load  which 
seldom,  if  ever,  covers  the  entire  beam,  and  the  designer  will 
have  to  use  his  best  judgment  as  to  what  constitutes  proper 
practice  in  each  individual  case.  It  is  hardly  necessary  to  say 
that  in  other  lines  of  engineering  the  most  dangerous  condition 
is  always  considered  in  making  the  calculations  as  a  matter  of 
course,  and  there  is  no  reason  why  other  professional  ethics 
should  prevail  when  dealing  with  reinforced  concrete. 


TRANSVERSE  STRESSES.  —  U-BARS 


101 


In  Figure  107  the  moment-curve  is  shown  corresponding  to 
a  uniformly  distributed  load  covering  the  entire  span.  The 
maximum  moment  is  taken  as  unit,  and  the  several  ordinates 
of  the  curve  are  given  under  the  assumption  that  there  is  no 
continuity.  The  reinforcement  must  be  made  to  conform  to 


n      12 


.75 


.89 


1X0 


.97 


FIGURE  107. 

this  curve  as  closely  as  possible,  hence  we  see  that  at  points 
3  and  9,  only  f  of  the  total  number  of  bars  is  required,  at  2  and 
10,  slightly  more  than  J,  and  less  than  J  is  required  at  points 
1  and  11.  The  quota  of  bars  not  required  may  and  should  be 
bent  up  at  the  points  specified,  provided  that  no  other  kinds  of 
loading  can  occur.  In  Figure  108  the  corresponding  curve  is 


FIGURE  108. 

shown  when  the  beam  is  considered  as  continuous,  with  q  =  10. 
63.  The  entire  theory  outlined  for  the  calculation  of  the 
U-bars  is  based  upon  the  assumption  that  sliding  of  the  steel 
cannot  take  place.  In  such  cases  where  the  anchorage  beyond 
or  at  the  supports  is  insufficient  to  prevent  sliding  of  the  main 
tension  bars  the  factor  of  reduction  must  be  decreased,  so  that  a 
correspondingly  larger  amount  of  vertical  reinforcement  is  used 
for  the  U-bars.  In  the  present  state  of  our  knowledge  this  must 
be  taken  care  of  by  judgment  alone,  there  being  no  way  of  cal- 
culating a  beam  with  inefficient  anchorage.  It  must  here  be 
sufficient  to  point  to  the  fact  that  the  U-bars  retard  the  sliding 
of  the  reinforcement,  and  that,  for  that  reason,  light  U-bars  should 
always  be  used  even  in  cases  where  the  theoretical  considerations 
show  that  they  may  be  dispensed  with.  This  applies  particularly 
to  rectangular  beams. 


102  REINFORCED  CONCRETE  BUILDINGS 

64.  Spacing  of  the  U-Bars.     It  will  be  noted  that  the  entire 
line  of  argument  advanced  in  the  preceding  paragraph  is  based 
principally  upon  the  inability  of  the  concrete  to  resist  tensile 
stresses,  and  that  the  entire  problem  finally  resolves  itself  into 
one  of  tension  carried  entirely  on  the  steel,  and  compression 
carried  entirely  on  the  concrete.     The  word  "  shear  "  is  referred 
to  incidentally  only,  and  this  is  a  natural  consequence  of  the 
fundamental   principle   of   disregarding   the   tensile   stresses   in 
the  concrete.     As  this  development  leads  to  rather  important 
results,  it  may  be  well  to  consider  these  matters  a  little  more 
in  detail. 

65.  Figure   109  shows  the  simplest  conceivable  system  of 
material   units,  i.e.,  three   particles,  A,  B,  and  C.      Whatever 

the  nature  of  the  force  uniting  these  par- 
tides,   if  the  particle  C  is  moved  to  the 
position  D  through  the  influence  of  some 
\        external  force,   the  displacement  CD  rep- 
.  \       resents  in  all  cases  the  result  of  the  influ- 

£__ _\;     ence  of  that  force  and  is  called  the  "shear 

A  B    deformation "     if    parallel  with   the    line 

FIGURE  109.  .  „       T,     .  ,.,  ,  ,, 

AD.     It    is   readily    seen,    however,   that 

the  more  direct  and  more  readily  understood  deformations 
are  (1)  the  lengthening  of  AC  to  AD,  and  (2)  the  shortening  of 
BC  to  BD.  Hence  this  shear  deformation  CD  is  nothing  but 
the  resultant  of  the  deformations  along  the  original  lines  AC 
and  BC,  and  we  perceive  that  even  in  the  most  complicated 
system  of  particles  any  deformation  may  be  reduced  to  a  sys- 
tem of  lengthenings  and  shortenings,  that  is,  tension  and  com- 
pression, if  we  speak  of  stresses  instead  of  deformations.  The 
word  "  shear,"  therefore,  has  no  real  or  material  meaning, 
except  as  a  pure  figure  of  speech  to  express  in  one  short  word 
a  rather  intricate  condition  of  tensile  and  compressive  rela- 
tions, in  precisely  the  same  manner  as  the  word  "  bending 
moment  "  is  used  to  indicate  a  mathematical  conception  of  the 
mutual  condition  of  a  number  of  forces  acting  upon  a  beam. 
Needless  to  say  that  nobody  has  ever  seen,  or  will  ever  see,  a 
bending  moment  in  the  realm  of  things  as  they  are,  and  that 
whoever  undertakes  to  explain  the  so-called  "  shear  stresses  " 
in  a  solid  body  will  ultimately  have  to  account  for  pure  tensional 
and  compressional  stresses. 


TRANSVERSE  STRESSES.  —  U-BARS  103 

66.  If  two  material  bodies  are  in  contact,  the  stresses  act- 
ing in  the  contact  surface  are  termed  frictional  stresses  which, 
as  far  as  the  materials  themselves  are  concerned,  are  compres- 
sive  stresses  with  no  possibility  of  accompanying  tensile  stresses 
in  the  direction  perpendicular  to  the  contact  surface. 

67.  Of  the  nature  and  extent  of  frictional  stresses  we  know 
next  to  nothing.     A  force  acting  parallel  with  the  contact  sur- 
face will  cause  sliding  of  one  body  in  relation  to  the  other;  if 
the  force  is  inclined,  the  sliding  becomes  increasingly  difficult 
as  the  angle  of  the  force  increases,  and  the  sliding  becomes 
impossible  when  the  angle  at  which  the  force  acts  exceeds  the 
"  angle  of  friction,"  which  has  a  definite  value  for  each  material, 
depending  in  part  upon  the  character  of  the  surface.     For  con- 
crete upon  concrete,  this  angle  appears  to  be  near  41°. 

68.  In  certain  types  of  reinforced  concrete  construction  the 
floor  beams  are  not  made  in  one  continuous  operation  with  the 
floor  slab  resting  upon  the  beams,  and  U-bars  or  similar  mechani- 
cal devices  are  then  resorted  to  in  order  to  tie  the  slab  and  stem 
together,  and  to  so  unite  them  that  they  may  be  considered 
as  acting  as  one  piece.     In  this  case,  the  slab  would  form  the 
upper  flange  of  a  T-beam,  and  in  order  to  insure  this  action, 
sliding  between  flange  and  stem  must  be  prevented.     Figure 
110  represents  a  portion  of  a  beam,  the  lines  AC,  CD,  and  CB 

c          P 

'Mf ;|i          *"  |, \M 


FIGURE  110. 

indicate  the  directions  of  the  principal  stresses.  If  now  the 
line  of  diagonal  compression  BC  is  inclined  so  that  the  angle 
BCD  is  less  than  the  angle  of  friction,  the  flange  would  slide 
in  relation  to  the  stem,  on  account  of  the  joint  along  the  line 
MM;  —  the  U-bars  AC  and  BD  would  resist  this  tendency  by 
virtue  of  their  "  shear  "  resistance  (and  this  resistance  we  know 
is  very  small,  and  cannot  exceed  the  compressive  edge  resist- 
ance of  the  concrete;  see  Figures  111,  112,  where  the  black  areas 
indicate  the  crushed  concrete).  If,  on  the  other  hand,  the  angle 
BCD  is  larger  than  the  angle  of  friction,  then  there  can  be  no 


104 


REINFORCED  CONCRETE  BUILDINGS 


sliding,  and  therefore  no  shear  stresses  on  the  U-bars,  which 
will  act  directly  in  tension  as  described  above.  The  rule  derived 
from  this  argument  may  be  briefly  expressed  thus:  The  spacing 
of  the  U-bars  must  not  exceed  the  depth  of  the  beam,  in  which 
case  the  angle  of  forces  would  be  about  45°. 


FIGURE  111. 


FIGURE  112. 


69.  If  we  now  turn  to  the  T-beam  manufactured  in  one 
continuous  operation,  where  no  separation  exists  between  stem 
and  slab,  we  note  that,  theoretically  at  least,  this  beam  is  in 
the  same  condition  as  the  one  just  considered,  owing  to  the  orig- 
inal assumption  whereby  the  tensile  stresses  in  the  concrete 
are    considered    as   non-existing.     Each    and   every   horizontal 
stratum  must  be  considered  as  isolated  and  influenced  by  its 
neighbor   through    the   medium    of   frictional    resistance    only, 
and  the  direction  of  the    diagonal  compression  must  be  such 
that  no  sliding  can  take  place.     The  same  rule  must  therefore 
be  imposed  in  this  case. 

But  this  rule  gives  the  maximum  spacing  possible:  owing  to 
the  usual  considerations  of  a  margin  of  safety,  the  spacing  must  be 
made  smaller,  and  we  would  therefore  recommend  that  the  spacing 
of  the  U-bars  must  in  no  case  exceed  one-half  the  effective  depth  of 
the  beam. 

70.  Tensile  Stresses  in  Concrete  Disregarded.     The  ready- 
made  reinforced  concrete  beam  formulas  now  in  common  use 
are  derived  under  the  apparent  assumption  that  the  steel  rein- 
forcement takes  all  the  tensile  stresses,  and  this  is  also  the  case 
in  this  book.     In  reality,  we  cannot  wholly  disregard  these  ten- 
sile stresses  in  the  concrete,  or,  at  least,  we  cannot  deny  their 
existence,  because  if  we  did,  we  would  also  rob  the  concrete 
of  its  cohesion,  and  we  would  have  a  granular  mass  such  as  sand 
or  crushed  stone,  wholly  unsuitable  for  our  purpose.     The  true 
statement  is  that  we   disregard  the  tensile  stresses  in  certain 
directions  and  for   certain  purposes.     In   this   book,   we   have 
considered  the  concrete  as  fractured  vertically  along  the  planes 


TRANSVERSE  STRESSES.  —  U-BARS  105 

of  the  U-bars  (1)  because  the  cracks  in  probability  will  appear 
in  the  weakest  plane,  there  being  less  concrete  to  resist  the  ten- 
sion where  the  concrete  is  displaced  by  the  steel  of  the  U-bar; 
(2)  because  the  U-bar  encircling  the  main  tension  rod  in  a  meas- 
ure acts  as  a  washer  on  the  rod,  causing  the  somewhat  resilient 
concrete  to  crack  immediately  behind  this  point  of  gripping; 
and  (3)  because  such  tests  as  throw  any  light  upon  the  location 
of  the  cracks  indicate  that  they  occur  very  largely  at  just  these 
points. 

71.  NOTE:  For  the  gripping  action  of  a  loose  U-bar  encir- 
cling the  tension  rod,  see  Morsch,  page  47. 

For  the  location  of  the  cracks,  see  the  same  book,  page  155. * 

72.  We  have  also  considered  the  stem  of  our  T-beam '  as 
composed  of  horizontal  layers  acting  upon  one  another  by  con- 
tact only,  and  thereby  determined  the  spacing  of  the  U-bars. 
But  between  these  vertical  and  horizontal  lines  of  weakness, 
we  have  assumed  the  concrete  to  be  solid.     Hence,  we  have 
assigned  to  the  concrete  a  certain  amount  of  tensile  resistance 
in  certain  locations  and  directions. 

73.  It  follows  that  with  increasing  loads  the  compressive 
stresses  in  the  beam  do  not  increase  as  rapidly  as  the  load, 
especially  not  in  beams  where  the  slab  and  the  stem  are  sep- 
arately   manufactured.     In    such    beams,    the    compression    at 
rupture  must  in  many  cases  be  uniformly  distributed  over  the 
entire  compressive  zone,  and  we  find  here  the  explanation  of 
the  fact  sometimes  observed  that  the  compressive  strength  of 
concrete  is  much  higher  in  a  beam  test  than  in  a  cube  test.    An 
analysis  of  these  conditions  would  be  interesting  and  of  great 
value  practically. 

74.  Details    of    Reinforcement.     The    various    arguments 
advanced  above  will  lead  to  rational  design  of  the  steel  if  con- 
sistently applied,  and  there  is  but  little  new  to  add.     The  great 
principle  in  all  beam  construction  is  that  there  is  a  compres- 

1  Morsc^i:  Concrete  Steel  Construction,  1909.  While  the  cracks  do  not 
all  occur  at  the  U-bars,  the  tendency  is  fairly  pronounced,  especially  in  the 
beams  with  U-bars  in  one  half  only,  see  Figure  149,  Beam  V;  Figure  153, 
Beam  VIII;  Fig.  154,  Beam  IX;  Fig.  157,  Beam  X;  and  compare  the  cracks 
in  the  U-bar  end  with  those  of  the  other  end  of  the  same  beam.  The  draw- 
ings of  all  these  beams  show  them  just  before  final  collapse,  while  our  calcu- 
lations have  reference  to  a  much  earlier  stage,  viz.,  under  the  working  load, 
or  at  the  most  a  load  not  more  than  twice  the  working  load. 


106  REINFORCED  CONCRETE  BUILDINGS 

sion  and  a  tension,  separate  from  one  another,  but  with  hori- 
zontal projections  of  equal  intensity  or  magnitude,  provided 
the  loads  are  vertical.  Whatever  the  arrangement,  the  com- 
pression and  tension  must  ultimately  meet  one  another  and 
annihilate  one  another,  whether  this  takes  place  gradually  by 
increments,  as  in  the  plate  girder  of  constant  depth;  or  in  one 
operation,  as  in  the  King  truss,  where  the  tension  chord  meets 
the  compression  chord  at  the  ends  of  the  beam;  or  in  a  number 
of  places,  all  well  defined,  as  in  the  Howe  truss.  We  have  seen 
that  the  rectangular  beam  is  somewhat  similar  to  the  King 
truss,  and  that  the  T-beam  is  very  similar  to  the  Howe  truss; 
we  have  also  pointed  out  that  the  theory  of  stress-transmission 
by  gradual  increments  is  not  tenable  under  high  loads  owing 
to  the  slight  tensile  resistance  of  the  concrete.  We  must 
assume  that  the  sooner  the  compression  and  the  tension  are 
brought  to  annihilate  one  another  the  better  will  our  beams 
withstand  the  loads,  hence  the  necessity  of  bending  the  rods 
up  as  soon  as  possible,  and  the  desirability  of  closely  spaced 
U-bars.  A  simple  and  effective  way  of  bending  the  bars  is 
shown  in  Figure  113.  The  point  of  bending  should  be  deter- 


FIGURE  113. 

mined  by  the  bending  moment,  so  that  there  is  steel  enough  to 
meet  the  requirements  at  all  points.  In  this  beam  and  the  fol- 
lowing we  must  suppose  that  there  are  some  straight  bars,  but 
these  are  not  shown  in  the  figures.  Hence  the  principal  stresses 
in  Figure  113,  disregarding  the  straight  bars,  are:  a  constant 
compression  along  the  slab,  a  constant  tension  in  the  rod,  and 
certain  vertical  resultants.  The  rod  has  a  curve  under  the  load 
A,  against  which  the  concrete  is  pressing.  The  resultant  of 
all  these  pressures  should  go  through  the  point  of  application 
of  A,  hence  the  rod  should  be  bent  to  a  circle  with  center  in  the 
point  of  application.  The  same  applies  to  the  reaction,  B,  and 
in  addition  the  rod  should  be  extended  beyond  the  support  to 
develop  the  full  adhesive  resistance. 

A  somewhat  more  complicated  method  is  shown  in  Figure  114 


TRANSVERSE  STRESSES.  —  U-BARS  107 

where  there  are  two  systems  of  bent  rods  (aside  from  some 
straight  ones).  The  "  first  "  rod,  AC,  is  curved  under  the  load 
P  for  the  reasons  explained  above;  in  addition,  the  resultants 
C  and  D  must  be  made  to  meet  one  another  in  the  same  point 
and  with  the  same  direction  and  same  force.  Hence  the  num- 
ber of  rods  in  each  chord  should  be  the  same.  The  length  of 
rod  in  compression  flange  ab  should  be  sufficient  to  develop  the 
full  strength  of  the  bond,  in  the  same  way  as  for  the  "  second  " 


\  \ 

FIGURE  114. 

chord  over  the  point  of  support.  The  slope  or  angle  of  the  bent 
bars  would  seem  to  be  of  no  importance;  but  many  authorities 
are  of  another  opinion  and  recommend  an  angle  of  about  45 
degrees.  (In  practice  the  bars  are  seldom  bent  to  such  large 
radii  as  shown  in  Figure  114,  this  diagram  being  purposely 
exaggerated.) 

75.  The  shape  of  the  U-bars  should  be  as  shown  in  Figure 
102a,  6,  with  curved  top  and  bottom,  and  hooked  over.     The 
downward  projection  of  the  end  makes  it  easy  to  support  the 
U-bar  on  the  form  work,  and  the  entire  U-bar  is  firmly  anchored 
against  sliding,  both  top  and  bottom :  the  top  on  account  of  the 
curves,  the  bottom  because  it  passes  around  the  reinforcement. 
The  direction  of  the  U-bar  should  be  vertical.    The  sloping  or 
slanting  U-bar  is  said  to  strip  the  concrete  away  from  the  ten- 
sion rod,  as  we  might  expect  if  our  theory  is  correct,  and  it  does 
not  give  as  efficient  reinforcement  in  the  small  cantilevers  as 
the  vertical  U-bar.     Round  U-bars  appear  to  be  better  than 
flat  bars;  but  there  is  a  great  amount  of  information  along  this 
and  similar  lines  which  will  have  to  be  furnished  before  rein- 
forced concrete  design  can  be  perfected.     But  our  lack  of  in- 
formation in  this  and  similar  cases  is  not  different  from  that 
existing  in  other  lines  of  engineering. 

76.  When  we  now  finally  combine  all  these  elements  to  one 
beam,  Figure  115,  we  have  a  structure  of  a  very  complicated 
nature,  and  we  must  ask  ourselves  if  all  these  stresses  can  travel 
through   and  between  one  another  as  here   assumed  without 


108 


REINFORCED  CONCRETE  BUILDINGS 


upsetting  our  calculations  and  assumptions  entirely.  To  this 
we  must  answer  that  we  do  not  know,  but  if  we  compare  our 
problem  with  those  met  in  other  lines  of  engineering  we  must 
admit  that  there  is  no  fundamental  difference  between  the  diffi- 
culties. Thus  a  combination  of  two  simple  Pratt  trusses  is 
treated  as  if  the  two  trusses  were  really  present  individually 
instead  of  combined  into  one  structure,  and  many  other  instances 


FIGURE  115. 

could  be  cited  to  show  that  we  often  have  to  dissolve  a  struc- 
ture into  its  apparent  elements  in  order  to  solve  its  problems. 
Assuming  the  reinforced  concrete  beam  to  be  similar  to  a  Howe 
truss,  as  here  proposed,  seems  to  be  no  more  of  a  mistake  than  to 
assume  the  connections  in  a  riveted  truss  to  be  frictionless, 
movable  joints.  But  the  approximations  made  in  steel  con- 
struction are  so  old  that  they  seem  almost  part  and  parcel  of 
the  art,  while  the  comparatively  new  assumptions  made  for 
reinforced  concrete  have  hardly  had  time  to  solidify,  and  they 
are  therefore  supposed  to  be  of  a  more  questionable  nature 
than  the  older  ones,  which  have  indeed  had  the  profit  of  the 
test  of  time.  Yet  there  is  a  number  of  reinforced  concrete 
buildings  about  thirty  years  old  which  stand  up  as  well  as  any- 
body could  wish,  and  the  modern  steel  sky-scraper  is  of  no  older 
date. 


CHAPTER  VIII 


APPLICATIONS  OF  THE  BENDING   THEORY 

77.  Continuity  of  Reinforced  Concrete  Beams.  The  dif- 
ference between  the  beam  with  simple  supports  and  the 
continuous  beam  is  that  the  continuous  beam  is  subject  to  a 
" reverse"  bending  moment  over  the  support,  while  in  the  simple 
beam  there  is  no  such  reverse  moment.  The  cantilever  beam 
is  an  example  of  the  beam  in  which  only  reverse  moments  exist, 
and  as  we  have  found  it  feasible  to  construct  reinforced  con- 
crete cantilevers  we  cannot  deny  that  continuity  may  exist  in 
reinforced  concrete  beams.  In  fact,  unless  special  precautions 
are  taken  to  eliminate  reverse  moments  over  the  supports,  we 
know  that  continuity  must  exist  and  should  be  taken  into  ac- 
count. The  question  is  then:  to  what  extent  are  the  ends  of 
a  reinforced  concrete  beam  restrained?  When  this  question 
is  answered  we  must  make  the  beam  strong  enough  to  resist 
the  bending  moment  at  the  column,  and  then  it  is  a  matter  for 
further  investigation  to  decide  in  how  far  the  beam  is  actually 
benefited  by  the  restraint  to  such  extent,  that  the  moment  at 
the  middle  of  the  beam  may  be  reduced. 

In  Figure  116  a  beam  is  shown  in  which  the  ends  are  per- 


p   Ibs.    lin.  foot 


FIGURE  116. 

fectly  restrained,  and  where  the  uniform  load  covers  the  entire 
span.     The  bending  moments  are  over  the  supports: 

MA  =  MB  =  &  pi2 
at  the  center:  Me  =  aV  pi2- 

Hence        MA  +  Mc  =  MB  +  Mc  =  (TV  +  A)  P?  =  i  P?; 

109 


110  REINFORCED  CONCRETE  BUILDINGS 

or:  the  total  amount  of  bending  moment  to  be  taken  care  of  in 
the  beam  with  "  built  in  "  ends  is  the  same  as  in  a  simply  sup- 
ported beam.  The  bending  moment  carried  by  a  reinforced 
concrete  beam  is 

m  —  a  constant  X  bd2  (Formula  8); 

hence  for  constant  depth  the  allowable  bending  moment  is 
directly  proportional  to  the  width  6.  At  C,  Figure  116,  the 
width  is  =  6,  but  at  the  support  where  the  reverse  moment 
must  be  taken  care  of,  the  width  of  beam  is  only  that  of  the 
stem  =  n.  Hence  if  we  assign  a  moment  Mc  to  the  middle  of 
the  beam,  the  end  will  only  carry  a  moment 

MA  = 

so  that  MA  +  Mc  =  \MC  +  Mc  =  \  pi2', 

u  o 


b       4 

we  have  MC  =  %  •  i  •  pi2  =  iV  pi2 

and  MA  =  MB  =  I-  TV  pi2  = 

T,  n       1 

For  6  =  6 

we  have  Mc  =  f  •  1  •  pi2  =  -—  pi2 

and  MA  =  MB  =  J  -  -&  pi2  =  A  pi2- 

The  moment  at  the  center  of  the  span,  in  the  case  of  a  T-beam, 
will  therefore  be  about 


and  at  the  end  ^  pi2. 

i  4U 

If  greater  depth  is  provided  near  the  support  the  reverse  moment 
may  be  increased  and  the  moment  at  the  center  of  the  span  may 
be  decreased  a  corresponding  amount.1  In  Europe  it  is  quite 

Attention  is  called  to  the  obvious  fact  that  no  degree  of  "restraint" 
can  be  allowed  at  wall  ends;  this  is  especially  true  for  beams  resting  in  brick 
work. 


APPLICATIONS  OF   THE  BENDING  THEORY 


111 


common  to  make  the  beams  and  girders  deeper  at  the  columns; 
in  America  the  beams  and  girders  are  usually  of  the  same  depth 
throughout.  The  American  practice  is  to  be  preferred,  because 
the  continuous  effect  depends  entirely  upon  the  stiffness  of  the 
supports:  the  slightest  yielding  of  the  footings,  or  even  the  com- 
pressibility of  the  columns  may  destroy  the  continuity  entirely, 
and  too  much  dependence  upon  the  continuous  effect  may  lead 
to  serious  trouble. 

In  a  slab,  the  depth  and  the  "  width  of  beam  "is  the  same 
at  the  middle  of  the  span  and  at  the  supports.  If  the  supports 
are  unyielding  there  may  be  some  excuse  for  allowing  a  higher 
degree  of  continuity  for  slabs  than  for  beams;  the  more  so 
because  tests  on  reinforced  concrete  buildings  point  distinctly 
to  such  effects.  Let  us  assume  a  degree  of  continuity  leading 
to  the  following  bending  moment: 

Me  =  -  p  I2. 
In  Figure  117  the  equivalent  system  of  construction  is  shown 


FlGURE    117. 


in  which  the  center  portion  is  considered  as  a  simple  beam 
resting  upon  cantilevers  of  span  R.     We  have  then 


(L  -  2  R)*  =  -  L\ 


hence     R  =  %  L    1  - 


(25) 


It  is  hardly  necessary  to  say  that  we  have  no  absolute  certainty 
that  the  slab  will  adjust  itself  to  conform  to  this  arbitrary  divi- 
sion of  the  bending  moment.  Yet  if  the  cantilever  is  made 
strong  enough  to  carry  its  load,  and  the  central  portion  strong 
enough  to  carry  its  share,  it  is  difficult  to  see  why  such  a  system 
should  not  be  perfectly  safe.  Other  assumptions  may  be  made 
and  carried  through  in  the  same  manner;  this  analysis  will  be 
used  later  for  the  calculation  of  the  "  mushroom  "  system  as 
invented  by  Mr.  Turner. 


112  REINFORCED  CONCRETE  BUILDINGS 

The  formulas  usually  given  for  continuous  beams  depend 
upon  the  factor  EL  The  value  of  /  for  a  reinforced  concrete 
beam  is  not  a  constant;  in  Article  79  we  shall  consider  this 
in  detail.  We  will  find  that  the  moment  of  inertia  depends 
upon  the  maximum  unit  stresses  in  the  point  considered,  and 
we  cannot  expect  these  stresses  to  be  uniform  throughout  the 
length  of  the  beam.  The  usual  application  of  the  formulas 
for  continuous  beams  presupposes  that  the  moment  of  inertia 
is  constant  throughout  the  length  of  beam,  and  we  cannot  there- 
fore apply  the  formulas  used  for  homogeneous  beams  to  the 
reinforced  concrete  beams  with  any  degree  of  certainty. 

78.  While,  then,  the  exact  degree  of  continuity  cannot  be 
determined,  continuity  does  nevertheless  exist  in  many  cases 
if  not  in  all,  and  the  stresses  thus  created  must  be  taken  care 
of.  These  are,  primarily,  tensile  stresses  over  the  supports, 
requiring  reinforcement  in  the  top  of  the  girders  over  the  col- 
umns, in  the  beams  over  the  girders,  in  the  slabs  over  the  beams. 
The  top  bars  may  be  loose  bars,  but  it  is  rather  difficult  to  main- 
tain such  bars  in  their  proper  position;  the  bent-up  bars  may 
be  utilized  as  top  reinforcement  with  good  results,  especially 
as  they  extend  a  distance  into  the  next  bay  in  any  case.  It  is 
evident  from  the  remarks  made  above  that  the  top  reinforce- 
ment over  the  support  should  not  be  less  than  25  per  cent,  of 
the  bottom  reinforcement;  usually  more  bars  are  bent  up,  but 
they  need  not  all  extend  as  far  beyond  the  support  as  the  bars 
designed  to  resist  the  reverse  moment.  For  a  uniform  load 
covering  the  entire  span,  the  point  of  inflexion  is  evidently  deter- 
mined by  the  Formula  25: 


R  =  JLfl 


v/S 


so  that  25  per  cent,  of  steel  mentioned  above  should  be  carried 
at  least  that  distance  out  from  the  center  of  the  support.  The 
bars  must  be  embedded  in  a  sufficient  amount  of  concrete  to 
develop  the  bond,  not  less  than  four  diameters  from  the  face  of 
the  concrete,  or,  if  closer  to  the  face  of  the  concrete,  they  should 
be  provided  with  inverted  U-bars.  The  stress  on  these  U-bars 
cannot  be  calculated,  —  it  is  their  presence  rather  than  their 
strength  which  benefits  the  beam. 

79.    Moment  of  Inertia.     The  moment  of  inertia  in  a  rein- 


APPLICATIONS   OF   THE  BENDING   THEORY 


113 


forced  concrete  beam  is  of  interest  only  because  certain  prob- 
lems connected  with  continuity  of  the  beam,  deflection,  etc., 
cannot  be  solved  except  through  a  knowledge  of  its  value.  The 
expression  given  below  is  of  indirect  value  only,  showing  that 
the  ordinary  formulas  for  continuity  do  not  apply  to  reinforced 
concrete  beams,  because  the  moment  of  inertia  is  not  a  con- 
stant for  the  length  of  the  beam,  as  is  usually  assumed  in  the 
solution  of  such  problems. 

The  moment  of  inertia  with  reference  to  the  neutral  axis 
may  be  found  as  the  sum  of  two  moments:  /i,  referring  to  the 
concrete  above  the  neutral  axis,  and  J2,  referring  to  the  steel 
below  the  neutral  axis,  the  concrete  below  this  line  being  dis- 
regarded as  usual.  We  have  then  (Figure  118) 


"a  sq. inches 


FIGURE   118. 


and 


Jl=l 

72  =  rad2  (1  -  x)2 


the  steel  being  considered  as  equal  to  ra  square  inches  of  con- 
crete. But  according  to  the  formulas  given  in  Articles  25  ff.  we 
have 

i  Cxdb  =  aS 

a_xdbC_        Mb       . 
2    S      2(l-x)  r' 
hence  (after  some  reduction) : 

7  =  /i  +  72  =  J  fed8  (1  -  J  a)  x2  (26) 

By  means  of  Formula  5  in  Article  27  the  expressions  derived 
above  for  d  and  st,  etc.,  may  now  be  verified.  The  real  import- 
ance of  Expression  26  is,  however,  that  it  shows  that  the  moment 


114 


REINFORCED   CONCRETE  BUILDINGS 


of  inertia  depends  upon  the  location  of  the  neutral  axis  which 
again  changes  with  the  stresses  in  the  various  points  of  the 
beam. 

80.  Beams  with  Reinforcement  in  the  Compression  Side. 
Sometimes   it   is   found   impossible   to   make   the   compression 
flange  of  the  beam  wide  enough  to  bring  the  concrete  stress  down 
to  the  allowable  maximum.     In  that  case  some  engineers  use 
compression  reinforcement,  but  as  a  matter  of  fact,  our  knowl- 
edge of  the  properties  of  such  beams  is  very  slight,  and  there 
is  grave  doubt  as  to  the  advisability  of  using  this  method  of 
construction  in  important  cases.     The  calculations  are  simple: 
to  the  bending  moment  sustained  by  the  beam  with  its  ordi- 
nary amount  of  reinforcement  is  added  another  bending  moment 
due  to  extra  reinforcement  in  top  and  bottom,  this  latter  cal- 
culated as  for  an  ordinary  steel  beam,  but  with  quite  low  stresses 
(not  to  exceed  10,000  Ibs./square  inch).     The  compression  bars 
must  be  laced  carefully  to  rthe  tension  bars,  but  under  any  cir- 
cumstances it  seems  hardly  possible  to  provide  properly  for  the 
excessive  shear  stresses  set  up  in  this  kind  of  beams.     A  steel 
I-beam  is  cheaper  and  better  in  places  where  this  kind  of  con- 
struction is  actually  necessary. 

81.  Combined    Bending    and    Compression.     The    section 
is  best  designed  by  trial.     In  the  case  of  an  arch  ring,  the  sec- 
tion is  rectangular,   and  the  symmetrical  reinforcement  is  of 

small  area  compared  with  the  concrete  sec- 
tion. The  bars  on  the  compression  side 
must  therefore  be  disregarded,  as  it  would 
require  too  many  hoops  to  make  this  rein- 
forcement effective  in  compression.  We  must 
select  the  depth  and  the  reinforcement  by 
judgment;  the  stresses  due  to  the  bending 
moment  alone  are  then  easily  found  by  For- 
mula 19  in  Article  49.  Let  Figure  119  rep- 
resent the  section;  let  Cm  and  Sm  denote  the 
stresses  just  found  due  to  the  moment  alone. 
If  now  in  Figure  119  gh  is  made  equal  to 
Sm/r  and  ef  is  made  equal  to  Cm,  then  the  line  gf  will  represent 
the  distribution  of  stresses  on  the  section  due  to  the  bending 
moment  alone.  The  stress  due  to  the  pressure  P  is  now  P/bd 
Ibs./square  inch;  this  is  represented  by  the  line  ik  parallel  with 


FIGURE  119. 


APPLICATIONS  OF   THE  BENDING  THEORY  115 

fg.  The  total  pull  in  the  steel  is  then  equal  to  the  area  of 
the  triangle  khl  times  the  width  6  of  the  section.  For  slabs  or 
arches  the  width  is  usually  taken  as  12 ".  The  final  concrete 
stress  ei  must  not  exceed  the  allowable  stress;  we  can  therefore 
arrive  at  a  preliminary  estimate  of  the  dimensions  required  by 
Formula  16,  assuming  a  materially  lower  "  allowable  stress  " 
for  the  concrete,  and  a  higher  stress  for  the  steel,  when  making 
the  first  trial. 

If  the  section  is  one  in  a  column  the  calculations  are  essen- 
tially different.  The  eccentrically  loaded  column  is  of  fre- 
quent occurrence;  in  fact,  few  columns  are  always  loaded 
centrally.  In  practical  cases  it  is  almost  always  impossible  to 
calculate  the  eccentricity  of  the  load,  and  elaborate  formulas  are 
therefore  of  little  or  no  use.  Tension  should  never  occur  in  the 
column;  if  there  is  tension  with  the  selected  arrangement  it  is 
better  to  change  the  lay-out.  The  percentage  of  steel  will 
always  be  much  greater  than  in  the  case  considered  above, 
and,  as  there  is  no  tension,  we  may  perhaps  calculate  our  col- 
umn as  a  homogeneous  section,  using,  however,  for  the  moment 
of  inertia  the  expression 


/  =  Ie  +  r  Is  (27) 

where  ]  Tc  ~ 

i.  ls  = 


Ic  =  mom.  of  inertia  of  concrete  alone, 
mom.  of  inertia  of  steel  alone. 


The  cases  where  the  condition  of  loading  can  be  ascertained 
with  any  degree  of  certainty  are  very  few  indeed,  and  when 
they  do  occur  the  bending  moment  is  likely  to  be  very  small. 
If  such  is  the  case  it  is  simpler  and  probably  as  correct  to  cal- 
culate the  column  as  a  pure  column,  using  a  correspondingly 
higher  factor  of  safety,  and  then,  if  necessary,  finally  investi- 
gate the  problem  assuming  the  neutral  axis  to  be  disposed  at 
the  center  of  the  section,  and  take  the  moment  of  inertia  with 
reference  to  the  center  line. 

82.  Chimneys.  As  an  example  of  approximate  methods 
of  calculating  a  piece  subject  to  bending  and  compression,  let 
us  consider  a  single  shell  chimney  of  uniform  thickness.  The 
diameter  d  (in  feet)  of  the  flue  is  given,  and  so  also  the  height  H 
(in  feet).  Let  the  outside  diameter  be  D  (in  feet);  the  area 
presented  to  the  wind  pressure  (w  Ibs.  per  sq.  ft.)  is  then  DH 


116 


REINFORCED  CONCRETE  BUILDINGS 


square  feet,  and  the  total  pressure  DHw  Ibs.     Hence  the  bend- 
ing moment  at  the  base   (the  overturning  moment)  becomes 
DHw  times  J  H  =  \  wDH2  Ibs.  X  feet. 

If  now  the  total  allowable  compressive  stress  on  the  concrete 
is  C  Ibs./sq.  in.  and  the  compressive  stress  due  to  the  weight 
of  a  column  of  concrete  1"  square  and  H  feet  high  is  (approxi- 
mately) H  Ibs./sq.  in.,  then  the  compressive  stress  due  to 
the  overturning  moment  must  not  exceed  C  --  H  Ibs./sq.  in. 
Assuming  the  neutral  axis  to  go  through  the  center  of  the  sec- 
tion, which  indeed  is  not  true,  and  disregarding  further  the  bene- 
fit derived  from  the  steel  in  the  compressive  side  (which  is  on 
the  safe  side),  the  moment  of  inertia  of  the  ring  is 


hence 


(C  -  H)  •  144  = 


64 


(D4  -  d4) 


D 
2 


which,  when  solved,  gives  the  outside  diameter 


D 


J  » 

V  - 


| 


187T  (C  -  H)   '    V  V187T  C  -  H, 
and  the  tension  per  inch  of  circumference  becomes 

\  (C  -  2  H)  (D  -  d)  Ibs. 

83.    Footings.     In  Figure  120,  2R  (inches)  denotes  the  side 
of  the  footing,  2r  (inches)  the  side  of  the  column.     The  bending 


FIGURE  120. 


moment  on  side  ab  (considering  the  footing  as  a  cantilever- 
slab)  corresponds  to  the  loaded  area  dabc.  We  have,  for  a 
load  p  Ibs./sq.  in. : 

Load  dcef  =  (R  —  r)2Rp;    arm  of  bending  moment  around 
ef  -  i  (R  -  r). 


APPLICATIONS  OF   THE  BENDING   THEORY 


117 


Hence  bending  moment 

A  =  pR  (R  -  r)2; 
load  aed  plus  bcf  =  (R  —  r)2  p ;  arm  of  bending  moment  around 

ef  =  402-0. 

Hence  bending  moment 

B  =  I  p  (R  -  r)3 

The  total  bending  moment  due  to  the  area  abed  is  then  the 
difference  between  A  and  B; 


and  the  depth  of  footing  becomes,  according  to  Formula  8,  for 
a  width  of  beam  2r  =  b 

'™  *--'-v*/«+1I 


-R-rJp 

ci    VG 


to  which  corresponds  a  pull  s,  in  the  steel,  for  the  distance  ab 
st  =  77:  •  2  r  • 


1- 


It  is,  however,  quite  necessary  to  provide  reinforcement  for 
the  portions  ae  and  bf ;  for  this  reason  the  amount  found  above 
may  be  multiplied  by  a  factor  estimated  at  about  2,  which 
gives : 

s  =  C^rd  (29) 

for  each  layer  of  steel  (Figure  121).     The  radius  of  the  column 

i 


314 

FIGURE  121. 

should  be  made  as  large  as  possible,  because  a  material  saving 
in  depth  of  footing  is  obtained  thereby;  usually  the  column 
must  have  an  enlarged  base  for  other  reasons  as  well.     In  Ar- 
ticles 14  and  15  we  found  the  cross-sectional  area  of  column: 
X  =  1400  F  for  a  hooped  column 
X  =  1060  F  for  a  plain  reinforced  column, 


118 


REINFORCED   CONCRETE  BUILDINGS 


so  that  the  average  pressure,  under  the  conditions  assumed,  is 
1400  and  1060  Ibs./sq.  in.,  respectively.  With  higher  per- 
centages of  reinforcement  these  pressures  may  become  mate- 
rially higher;  the  column  base  is. therefore  enlarged  so  that  the 
pressure  on  top  of  the  footing  does  not  exceed  the  allowable 
unit  pressure,  and  a  steel  plate  is  put  under  the  bars  in  order 
to  distribute  the  pressure  over  the  requisite  area.  According 
to  tests  by  Bach  this  allowable  pressure  may  be  somewhat 
increased,  owing  to  the  reinforcing  effect  of  the  surrounding 
concrete  of  the  footing,  but  it  does  not  seem  wise  to  exceed  say 
1000  Ibs.  per  square  inch.  The  thickness  of  the  plate  may  be 
approximately  determined  by  means  of  a  formula  by  Grashof: 

t  =  ^Vp  (30) 

where 

t   =  thickness  of  plate,  in  inches, 
r  =  radius  of  reinforcement  (=  |  of  diameter  of  column,  less  2"), 

in  inches, 
p  =  pressure  on  plate,  in  Ibs.  per  square  inch. 

The  dimension  Q  in  Figure  129  may  be  found  by  Formula 


FIGURE  122. 

28  above,  using  for  p  the  allowable  pressure  on  the  concrete. 

84.    Circular  Reinforcement  in  Plates.     The  circular  plate 
in  Figure  122  is  supported  on  a  central  column.     The  load  is 


APPLICATIONS  OF   THE  BENDING   THEORY  119 

uniformly  distributed  over  its  surface,  or  symmetrically  and 
continuously  disposed  along  the  circular  circumference.  A 
segment,  Oab,  will  then  be  subject  to  a  certain  bending  moment, 
which  moment  determines  the  depth  D  at  the  circumference  of 
the  column.  It  is  now  easy  to  show  that  when  the  load  is  uni- 
formly distributed  over  the  entire  surface,  the  same  formula 
applies  in  regard  to  depth  as  was  derived  above  for  a  square 
footing;  the  calculations  are  practically  the  same  and  need  not 
be  repeated  here.  When  the  load  is  distributed  along  the 
edge,  the  Expression  32  in  the  following  article  may  be  used. 

In  any  case,  we  will  assume  that  the  depth  is  known  in  the 
thickest  part  of  the  plate  (at  the  edge  of  the  column).  If  now 
the  distance  dc  is  one  inch  long,  we  have  by  Formula  9 

st  =  TV  c2  D, 

which  expression  leads  to  the  amount  of  steel  required  along 
the  radii,  the  bars  being  I"  apart  on  the  circumference  of  the 
column.  Imagine  now  that  all  these  radial  bars  be  cut  asunder 
over  the  top  of  the  column  —  disregarding  the  tensile  strength 
of  the  concrete,  each  bar  will  then  have  a  tendency  to  move 
outward,  so  that  if  a  steel  ring  surrounded  the  entire  plate, 
each  bar  would  exert  a  pressure  st  against  the  inner  face  of  the 
ring.  If  now  dc  equals  one  inch,  then  db  equals  one  inch  times 
R/r0,  hence  the  pressure  on  the  ring,  measured  in  pounds  per 
lineal  inch  of  circumference,  equals  st  X  r0/R.  The  tension 
on  the  ring  is  then 

SR  =  st'r^.R  =  ^czr0D.  (31) 

It  is  now  evident  that  the  ring  with  radius  R  and  designed  to 
resist  the  tension  SR  is,  mathematically,  sufficient  reinforce- 
ment, so  that  the  radial  bars  may  be  dispensed  with.  In  actual 
practice  this  is  somewhat  modified  owing  to  the  fact  that  con- 
crete shrinks  when  setting,  so  that  it  would  pull  away  from  the 
ring;  the  ring  would  therefore  exert  no  pressure  against  the 
concrete  until  a  substantial,  and  perhaps  dangerous,  deforma- 
tion had  taken  place.  But  when  the  ring  is  used  in  combina- 
tion with  a  radial  reinforcement  and  when  at  the  same  time  the 
depth  D  is  not  too  small  compared  with  the  radius,  say  D  larger 
than  ^  R,  then  the  ring  would  seem  to  be  a  very  efficient  rein- 
forcement. Direct  proof  of  this  statement  is  indeed  missing, 


120  REINFORCED  CONCRETE  BUILDINGS 

but  the  "  Mushroom "  floors  furnish  at  least  some  indirect 
information  in  this  respect,  as  they  probably  owe  their  strength 
in  a  great  measure  to  the  intelligent  use  of  circular  reinforce- 
ment. That  this  type  of  reinforcement  is  successful  in  other 
types  of  structures  may  be  seen  from  the  remarks  made  under 
"  columns,"  where  hoops  are  extensively  used  to  take  care  of 
stresses  somewhat  similar  to  those  existing  in  a  plate,  although 
the  plate  at  the  same  time  acts  as  a  beam.  Exact  analysis  is 
of  course  difficult  in  these  structures  which  border  upon  the 
class  where  reinforcement  may  sometimes  be  omitted  entirely :  it 
is  well  known  that  tapering  footings  are  often  constructed  with- 
out steel,  and  the  same  may  be  true  of  columns  in  special  cases. 

85.    Theory  of  plates.  —  The  "  Mushroom  "  System. 

A  reinforced  concrete  floor  without  beams  or  girders  is  first 
indicated  and  patented  by  Mr.  C.  A.  P.  Turner  of  Minneapolis. 
As  far  as  known  there  is  no  perfectly  satisfactory  way  of  finding 
the  stresses  in  constructions  of  this  kind,  although  buildings 
actually  constructed  on  this  principle  have  given  good  satis- 
faction, according  to  the  published  records.  The  stresses  must 
necessarily  be  of  a  very  complicated  nature,  especially  under 
concentrated  or  unsymmetrical  loads;  the  following  analysis 
does  not  pretend  to  solve  the  problem  in  anything  approaching 
a  general  way,  and  the  formulas  apply  only  in  case  the  entire 
building  is  loaded  with  a  uniformly  distributed  load.  The 
formulas  are  not  inconsistent  with  the  assumptions  made  for 
reinforced  concrete  construction,  and  they  are  therefore  pre- 
sumably a  step  in  the  right  direction.  It  is  well  known  that 
most  of  the  proposed  formulas  are  based  upon  the  theoretical 
strength  of  the  plates  with  equal  tensile  and  compressive  resist- 
ance, and  reinforced  concrete  does  not  possess  any  such  qualities. 

Figure  123  shows  the  general  scheme  for  a  floor  of  this  kind: 
the  floor  slab  is  simply  a  flat  plate  resting  upon  columns,  the  tops 
of  which  are  enlarged.  Let  the  uniformly  distributed  load  be  w 
Ibs./sq.  foot  and  the  span  I  feet.  The  slab  is  divided  into  six 
strips:  two  diagonal  strips  AD  and  BC,  and  four  strips  along  the 
sides  AB,  BD,  DC,  and  AC.  If  we  now  suppose  the  panel  to  be 
square,  the  load  on  each  of  the  crossing  diagonals  may  be  taken 
as  \w,  while  the  span  AD  =  BC  =  I  V2.  Then,  by  Formula  30 

for  AB:    d  =  lJ»  and   for   BC:    d=  ^ .  t/SL  1  - 1/«? 
Ci  V   q  a       V     q        a    V   q 


APPLICATIONS  OF   THE  BENDING  THEORY 


121 


so  that  the  depth  is  uniform,  and  our  problem  centers  around  the 
design  of  a  side  strip  like  AB.  The  notations  are  shown  in  Figure 
124,  where 


If 


FIGURE  123. 


FIGURE  124. 


L  inches  is  the  span  between  column  centers. 

p  the  load  in  Ibs./sq.  inch. 

R  the  radius  in  inches  of  a  certain  circular  plate. 

r0  the  radius  in  inches  of  the  support  under  the  plate,  here 
referred  to  as  the  "  cap." 

p  the  radius  of  the  column  in  inches. 

d  the  depth  of  the  slab  in  inches. 

D  the  depth  of  the  cap  in  inches. 

We  will  now  proceed  as  follows :  We  consider  the  floor  slab  as 
supported  on  the  edge  of  the  circular  plate  with  radius  R;  this 
plate  will  then  have  a  uniform  load  on  its  surface  and  a  concen- 
trated load  along  its  circumference.  Finally  the  "  cap  "  with 
radius  r0  will  be  designed  for  a  load  concentrated  on  its  circum- 
ference, disregarding  the  uniform  load  on  its  surface. 

The  total  area  of  the  floor  panel  between  the  four  column 
centers  is  L2  square  inches;  the  total  weight  corresponding  to 
this  area  is  pL2  Ibs.  The  area  of  the  circular  plate  with  radius 
R  is  irR2,  the  total  weight  on  same  ptrR2.  Hence  the  weight 
of  the  portion  outside  the  circular  plate  becomes  p  (L2  —  irR2)  Ibs. 


122 


REINFORCED  CONCRETE  BUILDINGS 


In  the  following  computations,  L2  is  always  large  compared  with 
TrR2  so  that  this  quantity  may  be  neglected,  which  is  also  on  the 
safe  side.  The  load  is  therefore  pL2,  and  as  the  circumference 
of  the  circular  plate  is  2  TrR,  the  load  per  lineal  inch  of  circum- 


ference  becomes 


producing  a  bending  moment  equal  to 


_ 

27TR 


(R  —  r0).     If  measured  per  lineal  inch  of  the  circumference 


of  the  cap  with  radius  r0  it  becomes,  by  multiplication  with  R/rc 
(Figure  125), 


«  p 


L2-7TR2 


27TK 


FIGURE  125. 


FIGURE  126. 


and  the  corresponding  depth,  for  6=1" 

L 


R  -  r0)  (32) 

for    the    circular    plate.     For    the   slab   portion   we   have,    by 
Formula  16: 

7       LT,         T.  '    l~ 

V  •  <33) 

q       c,\  q 

'The  value  of  r0  must  now  be  such  that  the  two  depths  become 
alike,  which  gives 

q 


rn  — 


(34) 


at  the  same  time,  the  value  of  R  is  determined  by  the  selected 
value  of  q  by  formula 


APPLICATIONS  OF   THE  BENDING   THEORY  123 

see  Article  67,  Formula  25.  The  depth  of  the  cap.  is  found  by 
the  formula  above,  as  the  load  again  is  pL2,  substituting  only  r0 
for  R  and  p  for  rot  hence 

D--J^-(r.-P).  (35) 

Ci  V     2-n-p 

According  to  Article  84  the  reinforcement  may  be  disposed  in  a 
ring  with  radius  r0;  the  tension  in  this  ring  becomes: 

ST  =  ^  c,r0D  (36) 

LA 

The  arrangement  is  shown  in  Figure  126,  where  the  thickness  t 
should  be  about  4"  so  as  to  cover  the  ring  thoroughly.  The  cap 
should  be  cast  in  one  piece  with  the  column,  but  there  is  no 
reason  why  a  joint  may  not  be  made  between  the  top  of  the  cap 
and  the  bottom  of  the  flat  portion  along  line  a  —  a  in  Figure  126. 
The  reinforcement  for  the  flat  portion  is  designed  as  for  any 
other  slab.  We  have  the  depth 


-  (33) 

q 

and  the  corresponding  pull  in  the  steel  $/  tons,  for  a  band  one 
foot  wide,  is  therefore,  according  to  (17) 

Sf  =  czd  (37) 

This  reinforcement  should  be  disposed  near  the  bottom  of  the 
slab  at  the  center  of  the  span,  and  near  the  top  over  the  columns. 
It  will  be  seen  that  this  leaves  a  considerable  space  around  the 
column  without  reinforcement  near  the  bottom,  which  should 
be  avoided.  We  may  therefore  follow  the  prevailing  practice 
and  bend  every  alternate  bar  up,  leaving  the  balance  of  the 
steel  straight  near  the  bottom.  The  reinforcement  over  the 
column  is  then  inadequate,  and  we  will  have  to  introduce  addi- 
tional steel  at  that  point;  —  if  we  decide  to  use  rings  we  may 
use  one  ring  with  radius  R,  the  strength  of  which  is  determined 
according  to  Article  84  by  the  formula 


(38) 

We  have  now  (Figure  127) 

Thickness  of  slab,  in  inches  d  =  —  y  -  (33) 


Pull  in  slab  steel  per  foot  wic 
Radius  of  upper  ring,  inches 

Tension  in  upper  ring,  tons 
Radius  of  cap-ring,  inches 
Tension  in  cap-ring 

Depth  of  cap,  inches 
q  is  the  factor  of  continuity, 

L-  r»            -  - 

ith,  tons    Sf  =  c<id                           (37) 
R  =  \L  (\  -  J^\        (25) 

SR=^Rc2d                    (38) 

r    —         ^         -7?                    (°£\ 

'o          r>          .      ,     ft                        (<J*J 
£  TT   —  (—    O 

Sr=-^c.sr0D                   (36) 

D  =  \  V^2^  (r°      s)     (35) 
q  =  10  to  16. 

n       -[ 

! 

H—  ^ 

^^^^M^M^M^MM^^^^^. 

teusiobsSt.        *  — 

^T             ™ 

FIGURE  127. 

All  dimensions  are  in  inches;  the  load  p  is  in  Ibs.  per  square  inch 
and  includes  both  the  given  live  load  and  the  weight  of  the  con- 
struction, itself. 


V  protection 


FIGURE  128. 


FIGURE  129. 


The  plan,  Figure  128,  shows  the  disposition  of  the  strips.     If 
we  draw  the  four  circles  with  radii  n,  and  if  n  =  1/5  of  L,  the 


APPLICATIONS  OF   THE  BENDING  THEORY  125 

outlines  of  the  strips  will  be  determined  as  tangents  to  the  circles, 
and  all  portions  of  the  slab  will  be  covered  with  reinforcement. 

It  is  evident  that  this  entire  treatment  cannot  lay  claim  to 
great  exactness,  as  the  stresses  probably  are  very  much  more 
complicated  than  here  assumed.  In  any  case,  the  solution  here 
given  is  correct  under  the  assumption  only  that  the  entire  floor 
is  covered  with  the  same  load  at  all  points.  The  results  are 
somewhat  in  accordance  with  current  practice  for  q  =  16,  so  that 
the  tests  made  on  actual  structures  of  this  kind  may  to  some 
extent  be  taken  as  circumstantial  evidence  of  the  soundness  of 
the  formulas,  if  not  of  the  argument  on  which  they  are  based. 

It  must  be  noted  that  these  formulas  do  not  apply  to  wall 
panels,  because  continuity  of  construction  does  not  obtain  at 
those  points,  while  also  the  arrangement  is  unsymmetrical. 
The  outside  bays  should  therefore  always  be  carried  on  girders 
and  beams  in  the  usual  way;  it  seems,  however,  that  in  some 
applications  of  this  type  of  floor  the  flat  construction  has  been 
carried  entirely  out  to  the  walls. 


CHAPTER   IX 

INITIAL  AND  ALLOWABLE  STRESSES 

86.  CALCULATION  of  the  "  initial  stresses  "in  reinforced  concrete 
structures  is  an  impossibility  with  the  data  on  hand  at  the  present 
time,  hence  it  becomes  impossible  to  combine  these  stresses  with 
those  considered  in  the  preceding  articles,  the  "  static  "  stresses. 
What  we  know  about  the  initial  stresses  is  due  chiefly  to  the 
careful  investigations  of  Considere;    the  results  may  briefly  be 
described  thus:  — 

87.  Concrete  Setting  in  Air  Shrinks,  the  more  so  the  richer 
the  concrete  is  in  cement.     If  this  shrinkage  can  take  place  unre- 
strainedly no  stresses  are  set  up ;  in  reinforced  concrete  the  steel 
will  naturally  counteract  the  shrinkage  so  that  the  steel  is  com- 
pressed and  the  concrete  put  in  tension.     This  rather  benefits 
the  beams,  as  the  tension  below  the  neutral  axis  is  disregarded  in 
any  case,  while  the  compression  in  the  steel  decreases  the  tension 
stresses  produced  under  load.     On  the  compression  side,   the 
tensile  stresses  in  concrete  due  to  shrinkage  counteract  the  com- 
pression produced  by  the  load,  so  that  on  the  whole  the  initial 
shrinkage  stresses  are  beneficial  in  beams.     In  the  case  of  a 
column  this  is  entirely  reversed,  as  the  initial  compression  in  the 
steel  must  be  added  to  the  compression  due  to  the  load,  while  on 
the  other  hand  the  compression  in  the  concrete  is  less  than  calcu- 
lated.    In  the  hooped  column  the  shrinkage  of  the  concrete  has 
some  influence  on  the  stresses  in  the  hoops :  —  a  higher  intensity 
of  lateral  pressure  is  required  in  order  to  bring  tension  on  the 
hoops. 

88.  Concrete  Setting  in  Water  Swells,  the  more  so  the  richer 
the   concrete   is   in   cement.     In   un-reinforced    concrete   these 
stresses,  if  restrained,  are  beneficial;    in  reinforced  concrete  the 
swelling  puts  the  concrete  in  compression  and  the  steel  in  tension. 
Usually   reinforced    concrete   members    set    in  the   air,  except 
footings,  sea-walls,  and  such  .structures;    it  must  therefore  be 

126 


INITIAL  AND  ALLOWABLE  STRESSES  127 

possible  to  keep  the  concrete  in  such  a  moderate  state  of  moisture 
that  no  initial  stresses  of  importance  are  set  up  during  the 
hardening  period.  Hence  the  necessity  of  sprinkling  the  concrete 
freely  for  the  first  two  or  three  weeks;  this  should  also  be  done 
liberally  in  order  to  furnish  the  setting  concrete  with  the  necessary 
water  to  make  the  chemical  action  in  the  cement  take  place  as 
required. 

89.  When  once  the  Concrete  is  Hard  and  Dry,  addition  of 
water  makes  the  concrete  swell;  these  variations  in  volume  are 
the  more  dangerous  the  less  cement  the  concrete  contains,  because 
the  stresses  produced  are  nearly  the  same  if  not  higher,  while  the 
resistance   against    tensile    stresses    is,   of    course,   less    in    the 
leaner  concrete.     It  is  important  to  keep  the  concrete  equally 
moist  all  the  time,  and  water  should  therefore  be  put  on  regularly 
and  frequently;    on  concrete  three  or  four  weeks  old,  and  dry, 
a  sudden  addition  of  large  quantities  of  water  may  prove  inju- 
rious.    If,  however,   the  concrete  has  been  test-loaded  so  that 
larger  stresses  have  existed  in  the  concrete  prior  to  the  wetting, 
no  change  in  volume  seems  to  take  place. 

90.  It  is  a  question  of  great  importance  to  settle  the  exact 
intensity  of  stress  in  a  reinforced  concrete  member  before  the 
load  is  put  on.     Only  then  will  it  be  possible  to  design  concrete 
structures  with  absolute  economy :    the  better  informed  we  are 
in  regard  to  the  distribution  of  stresses  in  any  given  case  the 
smaller  can  we  make  the  factor  of  safety.     It  seems  indeed  that 
the  shrinkage  stresses  are  large  enough  to  crack  reinforced  con- 
crete slabs,  even  sometimes  beams;   the  best  remedy  is  to  keep 
the  concrete  moist,  so  as  to  avoid  excessive  stresses,  and  to 
extend  the  bars  well  beyond  the  supports,  or  otherwise  anchoring 
the  bars,  to  prevent  sliding.     While  cracks  certainly  look  bad, 
this  kind  of  cracks  cannot  have  any  great  effect  upon  the  initial 
stability  of  the  structure,  because  we  assign  no  tensile  resistance 
to  the  concrete.     There  is  no  definite  or  conclusive  information 
available  to  the  writer  giving  data  on  the  durability  of  members 
cracked  in  this  way;  we  are  perhaps  justified  in  concluding  that 
no  bad  action  takes  place.     The  only  danger  seems  to  be  from 
corrosion  of  the  exposed  steel,  but  the  efflorescence  comes  to 
our  help  in  this  case,  frequently  filling  the  cracks  completely. 

91.  Temperature  stresses  do  not  usually  exist  in  unrestrained 
reinforced  concrete  members,  because  the  concrete  is  a  fairly 


128  REINFORCED  CONCRETE  BUILDINGS 

good  conductor  of  heat,  and  the  coefficient  of  expansion  is  nearly 
the  same  for  steel  and  for  concrete.  In  large  buildings,  tempera- 
ture expansion  may  indeed  cause  some  trouble,  because  the 
concrete,  in  expanding,  throws  the  columns  out  of  plumb  and 
causes  the  walls  to  turn.  Expansion  joints  are  frequently  made 
in  long  buildings;  in  later  years,  however,  expansion  joints  are 
not  used  as  much  as  they  were,  except  of  course  in  retaining 
walls  and  similar  structures  where  frost  and  heat  have  unchecked 
sway.  In  all  structures  such  as  arches,  continuous  bridge 
girders,  etc.,  a  serious  error  may  be  committed  by  disregarding 
the  temperature  stresses. 

92.  The  expansion  joint  as  used  for  plain  concrete  work  does 
not  interest  us  in  this  connection.     In  the  reinforced  concrete 
wall  it  is  indeed  questionable  whether  more  is  not  lost  by  giving 
up  the  continuity  than  by  having  a  few  fine  cracks  at  intervals. 
It  must  be  remembered  that  usually  the  expansion  joint  is  a 
point  of  discontinuity  in  the  steel  as  well  as  in  the  concrete,  and 
a  section  exposed  to  accidentally  higher  loads  or  stresses  than 
planned  loses  the  support  of  adjacent  sections  which  perhaps 
are  not  quite  so  overloaded.     In  any  case,  an  expansion  joint 
must  be  a  clean  joint  through  the  entire  body  of  the  concrete; 
simply  marking  off  of  the  surface  does  not  constitute  a  joint. 

93.  Allowable  Stresses.     The  analysis  given  in  the  preceding 
articles  applies,  in  so  far  the  mathematics  are  concerned,  to  any 
composite  material  having  properties  consistent  with  the  assump- 
tions made.     These  are,  as  will  be  remembered,  that  (1)   the 
concrete  has  no  tensile  resistance,  (2)  that  sections  plane  before 
loading  remain  plane  after  loading,  and  (3)  that  the  coefficient 
of  elasticity  remains  constant  up  to  the  point  of  loading  investi- 
gated.    In  addition,  a  number  of  assertions  have  been  made,  for 
instance  that  a  bond  exists  between  steel  and  concrete,  that 
concrete  has  a  lateral  expansion,  that  continuity  exists  in  rein- 
forced  concrete  beams,   and   other   similar   statements.     As   a 
matter  of  fact,  the  concrete  has  a  definite  tensile  resistance,  the 
coefficient  of  elasticity  is  not  a  constant  as  usually  determined, 
and  it  is  doubtful  whether  or  not  the  sections  remain  plane. 
That  the  bond  exists  cannot  be  doubted,  and  the  lateral  expan- 
sion as  well  as  the  continuity  are  well-established  facts.     The 
question  here  is  their  numerical  value,  without  which  we  can- 
not design  consistently  and  economically. 


INITIAL  AND  ALLOWABLE  STRESSES  129 

94.  The  allowable  stresses  are  used  in  the  design  for  the 
purpose   of  obtaining   an  ample   "  factor  of   safety."     At  the 
present  time,  the  relation  between  allowable  stress  and  factor 
of  safety  is  in  doubt,  and  it  will  probably  always  remain  so. 
The  reason  for  this  is  that,  even  with  all  the  materials  stored 
before  our  eyes  and  open  to  investigation,  the  strength  of  the 
concrete   cannot   be   predicted   with   certainty,   much   less   the 
bonding  strength.     Even  the  strength  of  a  given  cement,  mixed 
with  a  given  quantity  of  water  in  a  room  of  given  temperature, 
is  a  variable  quantity  as  reported  by  different  testing  laboratories. 
In  addition,  the  more  recent  experiments  made  on  reinforced 
concrete  specimens  show  that  their  ultimate  strength  is  not  an 
absolute  quantity,  but   depends  within  wide   limits   upon  the 
number  of  repetitions  of  the  load.     It  follows  that  a  very  large 
number  of  experiments  made  in  the  usual  way  —  loading  the 
specimen  once  only  —  are  misleading  in  their  results  and  cannot 
be  of  much  value  unless  compared  with  tests  in  which  a  large 
number  of  repetitions  of  the  load  have  taken  place. 

95.  In  spite  of  these  apparently  unsurmountable  difficulties, 
reinforced  concrete  design  is  at  present  established  on  a  fairly 
firm   basis,  especially  if   compared   with  design   involving   the 
use  of  other  engineering  materials.     The  strength  of  wood,  of 
natural  stone,  and  even  of  steel,  is  subject  to  doubt  in  many  cases. 
It  is  only  necessary  to  point  to  such  questions  as  those  connected 
with  the  strength  of  steel  columns  with  more  or  less  "  fixed  "  ends 
to  show  that  not  everything  is  settled  beyond  doubt,  and  many 
other  instances  could  be  cited.     But  we  have  to  bear  in  mind 
that  the  allowable  stresses  assigned  to  reinforced  concrete  are 
principally  established  through  practice  and  have  but  little  to 
do  with  the  laboratory  experiments.     The  actual  proof  of  the 
stability  of  reinforced  concrete  construction  is  furnished  by  the 
many  splendid  structures  erected  of  this  material,  and  only  to 
slight  degree  by  the  many  haphazard  experiments  made,  out  of 
which  most  any  kind  of  a  theory  could  be  construed.     The 
difference  between  concrete  as  actually  used  and  as  tested  in 
the  laboratory  is,  that  in  the  building  all  steel  rods  are  securely 
anchored  in  the  adjacent  span  (or  ought  to  be),  while  in  the 
laboratory  individual  beams  are  tested  without  any  arrangement 
to  secure  the  proper  sliding  resistance.     When  these  results  are 
analyzed  by  means  of  an  erroneous  shear-theory,  it  is  no  wonder 


130  REINFORCED  CONCRETE  BUILDINGS 

that  the  "  shear  "  resistance  today,  after  25  years  of  experi- 
menting, is  as  much  in  dispute  as  ever.  The  same  applies  to 
bonding  tests :  the  diameter  of  the  concrete  specimen  is  entirely 
disregarded,  yet  this  dimension  is  at  least  as  important  as  the 
length  of  embedment. 

96.  For  these  reasons,  the  allowable  stresses  are  not  taken  as 
a  certain  fraction  of  the  ultimate  strengths,  or,  if  they  were, 
that  fraction  would  not  necessarily  be  the  "  factor  of  safety." 
The  allowable  stresses  are  fixed  by  practice  grown  out  of  the 
accumulated  experience  of  many  years,  as  a  compromise  between 
the  conservative  designer  on  one  side  and  the  economist  on  the 
other  side.     They  have  no  meaning  whatever  unless  accompanied 
by  an  extensive  set  of  specifications  calling  for  certain  materials 
prepared  in  a  certain  way,  and  they  are  therefore  largely  a  local 
issue  to  be  determined  by  the  quality  of  obtainable  and  prevailing 
materials  in  each  locality,  coupled  with  and  correlated  by  the 
obtainable    engineering    supervision    prevailing    in    that    same 
locality.     This  supervision  is  always  necessary;  not  only  because 
the  temptation  to  "  save  "  may  be  too  great,  but  much  more 
because  the  intelligent  and  efficient  handling  of  the  concrete  is, 
in  reality,  the  factor  which  determines  the  final  factor  of  safety. 
It  is  the  duty  of  the  inspector  to  enforce  the  specifications  in 
letter  and  spirit;   it  is  not  less  the  duty  of  the  engineer  to  draw 
up  specifications  which  can  be  enforced,  and  at  the  same  time 
compel  the  use  of  first-class  materials.     It  must  not  be  understood 
that  good  inspection  alone  will  save  the  reinforced  concrete  job 
from  all  dangers:    willing  cooperation  between  contractor  and 
engineer,  inclination  on  the  owner's  side  to  pay  a  fair  price  for 
the  work,  and,  most  of  all,  a  full  understanding  of  the  why's 
and  wherefore's  are  indispensable. 

97.  With  first-class  materials,  it  is  customary  in  the  United 
States  to  use  the  following  allowable  stresses: — 

(a)  Columns.  The  allowable  stress  on  the  steel  is  dependent 
upon  that  used  for  the  concrete;  we  have  S  —  rC.  It  is  usual 
practice  to  take  r  =  15  or  20  for  columns;  the  latter  figure  pre- 
vailing. It  is  common  practice  to  allow  500  Ibs./sq.  inch  for 
concrete  when  the  columns  are  centrally  loaded;  when  a  small 
eccentricity  exists  which  cannot  be  estimated  in  figures  400  Ibs./ 
square  inch  may  be  allowed.  This  is  for  a  concrete  mixture 
having  a  mortar  base  of  one  part  of  cement  to  two  parts  of  sand; 


INITIAL  AND  ALLOWABLE  STRESSES  131 

according  to  the  size  of  the  column  and  of  the  aggregate  the 
proportion  of  stone  may  be  from  two  to  three  times  the  volume 
of  the  cement.  However,  when  first-class  materials  are  used  for 
a  first-class  job,  these  stresses  are  very  conservative,  and  600 
to  700  Ibs./square  inch  are  frequently  used  when  the  percentage 
of  steel  is  low.  It  stands  to  reason  that  large  amounts  of  steel 
should  be  avoided,  the  more  so  the  higher  the  stresses  on  either 
steel  or  concrete. 

(b)  Floors.     While  it  is  questionable  practice  to  use  different 
mixtures  for  different  parts  of  the  same  building,  it  is  frequently 
done  because  the  richer  mixture  makes  it  possible  to  decrease  the 
size  of  the  columns,  while  economy  calls  for  as  lean  a  mixture  in 
the  floor  as  may  consistently  be  used,  while  at  the  same  time 
the  leaner  mixture  is  less  liable  to  cracks.     Some  engineers  use 
therefore  as  lean  a  mixture  as  1 : 3 : 6,  corresponding  to  an  allowable 
stress  of  450  or  500  Ibs./sq.  inch,  while  600  Ibs./sq.  inch  is  used 
for  a  1:2^:5  mixture  and  700  for  a  1:2:4  mixture.     It  is  recom- 
mended to  use  the  higher  stress  and  the  richer  mixture,  and  avoid 
the  cracking  as  far  as  possible  by  liberal  sprinkling;   the  author 
does  not  consider  the  usual  mixture  of  1 : 2 : 4  as  being  very  good  for 
thin  reinforced  concrete  floors  and  would  prefer  1 : 2 :  3J  as  allowing 
a  greater  latitude  in  the  manipulation.     While  the  steel  stress  is 
fixed  in  its  relation  to  the  concrete  stress  in  columns,  there  is  no 
such  relation  for  the  floors.     It  is  customary  to  use  16,000  Ibs./ 
square  inch  for  low-tension  steel,  and  20,000  Ibs./square  inch  for 
high-tension   steel.     At    the   same   time,    proper    anchorage   is 
provided  for  by  extending  the  steel  bars  into  the  next  bay,  or 
by  hooking  the  bars  with  an  open  hook  at  the  end;  see  Article  8. 
The  higher  the  stress,  the  longer  should  be  the  embedment,  hence 
for  low-tension  deformed  bars  24  diameters  should  be  used,  and 
36  diameters  for  high-tension  deformed  bars.     Plain  bars  should, 
as  said,  have  an  additional  hook  on  the  ends  equal  in  length  to  6 
diameters.    It  is  prevailing  custom  to  make  r  =  15;  for  continuous 
construction  q  is  usually  taken  as  10,  while  for  non-continuous 
beams  8  is  used. 

(c)  Other  Structures.    Arches  are  usually  designed  by  calcula- 
tion of  the  bending  moments  and  thrusts  due  to  the  live  load, 
the  weight  of  the  structure  itself  and  the  fill  on  same,  and  the 
changes  in  temperature  existing  in  the  locality  where  the  arch 
is  to  be  built.     The  allowable  stresses  are  then  usually  taken  as 


132  REINFORCED  CONCRETE  BUILDINGS 

for  columns;  the  tension  in  the  steel  (if  any)  should  not  exceed 
10,000  or  12,000  Ibs./square  inch,  but  much  depends  upon  the 
care  and  method  of  analysis.  Some  engineers  consider  arbitrarily 
the  condition  where  one-half  of  the  arch  is  loaded  over  its  entire 
area;  it  is  by  no  means  certain  that  this  load  is  the  most  danger- 
ous. Where  the  analysis  is  based  upon  the  actual  maximum 
stresses,  the  allowable  stresses  may  be  somewhat  increased, 
especially  where  the  foundation  is  of  unyielding  nature,  as  bed 
rock,  and  the  abutments  are  of  sufficient  area  and  weight. 

Retaining  walls  may  be  considered  as  pieces  subject  to  bend- 
ing in  case  of  the  modern,  ribbed  construction.  Special  attention 
must  be  paid  to  the  proper  anchoring  of  the  tension  reinforcement, 
as  walls  of  this  kind  usually  are  built  on  the  cantilever  principle. 

Stand-pipes  and  water  pipes  subject  to  internal  pressure  re- 
quire low  stresses  in  order  to  become  water-tight  under  pressure; 
10,000  Ibs./square  inch  in  tension  on  steelshould  be  the  upper  limit. 
The  stability  of  the  finished  structure  depends  upon  the  integrity 
of  the  joints  between  the  hoops  belonging  to  the  same  circle 
or  spiral,  hence  the  thickness  of  the  concrete  and  the  length  of 
"  lap  "  are  most  important  factors.  The  concrete  should  be  at  least 
equal  in  thickness  to  10  diameters  of  the  embedded  steel  bars 
if  these  are  comparatively  heavy  and  spaced  comparatively  far 
apart,  say  8  to  12  inches;  if  light,  closely  spaced  bars  are  used 
the  thickness  should  be  increased  rather  than  decreased,  and 
special  pains  taken  with  each  joint.  The  best  way  is  to  break 
joints  whenever  possible  so  that  no  two  adjacent  joints  come  in 
the  same  plane,  to  keep  the  bars  to  be  joined  a  distance  apart 
equal  to  two  steel  diameters,  and  to  surround  the  joint  for  its 
entire  length  with  a  coil.  It  is  not  often  that  all  these  precau- 
tions are  taken  at  once.  It  has  been  found,  however,  that  stand- 
pipes  may  fail  by  the  concrete  inside  the  hoops  separating  from 
the  concrete  outside  the  hoops,  thus  destroying  whatever  bond 
may  have  existed  between  the  steel  and  the  concrete.  This 
danger  is  best  avoided  by  having  no  vertical,  concentric  planes 
of  weakness  in  the  concrete. 

98.  The  factor  of  safety  of  a  reinforced  concrete  structure 
depends  therefore  upon  the  selected  stresses  for  the  two  materials 
and  the  bond  stress  selected  for  anchoring  the  ends  of  the  bars; 
also  upon  the  selected  value  of  the%  ratio  r  between  the  coefficients 
of  elasticity  of  the  two  materials,  and  finally  upon  the  degree  q 


INITIAL  AND  ALLOWABLE  STRESSES  133 

of  continuity.  All  of  these  influence  the  safety  of  the  building, 
each  in  a  different  manner,  and  some  in  a  different  manner  at 
different  times.  It  has  therefore  been  found  impossible  to  estab- 
lish a  definite  "  factor  of  safety  "  for  reinforced  concrete  build- 
ings, but  we  know  that  the  carrying  capacity  of  a  building 
designed  as  here  described,  and  erected  in  a  first-class  manner, 
will  easily  carry  three  times  the  calculated  dead  and  live  load 
under  one  or  a  few  test  loads.  We  also  know  that  it  will  carry  a 
very  great  number  of  repetitions  of  twice  the  calculated  total  load, 
but  it  will  probably  not  carry  an  unlimited  number  of  repetitions 
of  three  times  the  calculated  total  load.  It  follows  that  a 
material  increase  in  the  allowable  stresses  suggested  is  dangerous 
at  the  present  time. 


PART   III 

PRACTICAL   CONSTRUCTION   OF  REIN- 
FORCED  CONCRETE   BUILDINGS 

BY  ERNEST  L.   RANSOME   AND  ALEXIS  SAURBREY 


CHAPTER  X 

MATERIALS  OF  CONSTRUCTION 
REQUIREMENTS  AND  TESTS 

Cement.  The  strength  of  concrete  depends  principally  upon 
the  quantity  and  quality  of  cement  used.  In  order  to  insure  a 
satisfactory  and  uniform  grade  of  cement,  the  shipments  are 
tested  according  to  rules  laid  down  by  the  "  American  Society 
of  Civil  Engineers,"  and  the  results  must  conform  to  the  require- 
ments of  the  standard  specifications  prepared  by  the  "  American 
Society  for  Testing  Materials."  Copies  of  these  publications 
may  be  obtained  from  the  societies  named,  but,  as  the  rules  are 
subject  to  variation,  the  specifications  are  not  printed  here. 

It  is  an  open  question  to  what  extent  test  reports  may  be 
depended  upon.  The  specifications  call  for  certain  numerical 
values  to  be  obtained,  but  it  is  now  well  established  that  no  two 
individuals  can  obtain  the  same  numerical  result  from  identical 
samples  of  cement  submitted  to  them.  The  reasons  for  this  are 
many;  for  instance,  atmospheric  conditions  may  be  different 
and  many  other  uncontrollable  factors  may  and  do  enter.  The 
greatest  trouble  is,  however,  that  the  manipulation  influences 
the  strength  of  the  test  piece,  and  no  two  experimenters  will 
handle  the  cement  mortar  in  identical  manner. 

Whatever  the  reasons  —  the  facts  remain.  On  the  following 
page,  Table  A  gives  the  results  of  cement  reports  made  in  various 
laboratories  on  identical  samples  of  cement:  Case  I  is  a  cement 
which,  while  somewhat  deficient  in  some  respects,  was  neverthe- 
less of  fair  quality,  yet  proved  to  be  extremely  quick  setting  on 
the  job.  The  letters  A,  B,  C,  etc.,  refer  to  the  parties  making 
the  test;  A,  B,  and  E  are  testing  laboratories  in  Cleveland, 
Ohio,  all  of  good  reputation;  C  is  instructor  in  cement  testing 
at  a  large  engineering  college;  D  is  a  concern  of  national  reputa- 
tion. The  reader  is  asked  to  compare  the  results  in  each  group 
and  then  draw  his  own  conclusions.  Table  B  gives  the  results 

137 


138 


REINFORCED   CONCRETE   BUILDINGS 


TABLE   A,  SHOWING    VARIOUS   TESTS   ON   CEMENT   BY    DIFFERENT   LABORATORIES 


-Lbs.  per  sq.  in.- 


Volume  Am. 
Soc.  C.  E. 


Strength,  neat      Strength,  1:3     • * »      Accelerated 

Tested  by          Initial.  Final.   100m.   200m.  24hrs.   7d.     28  d.     7  d.     28  d.     Air.  water.  Steam.    Boil. 


< — Fineness — ^ 


Standard    
CASE   I: 
A  
CASE    II: 
A 

1.0  to 
+0.30       10.0 

3.30         6.0 
30            50 

92          75 
94.4       82.6 
96  6       85  8 

175       500       600 
165       324       653 
371       679       771 

175         250 
159         307 
274         351 

OK       OK 
OK       OK 

OK 
OK" 

B 

1  55         30 

93  5       82  8 

179        

OK 

C 

1.35         4  30 

97.5       86  0 

273       613        — 

181 

OK          OK" 

CASE    III: 
A  

4.45         8.10 

94.9       81  7 

207       614        749 

195        293 

OK       OK 

OK 

B  

0.45         1.50 

93.0       81.5 

168       492       585 

134         391 

Soft 

CASE   IV: 
A 

3  50         5  10 

94  6       82  0 

267       602       697 

252         333 

OK       OK 

OK 

C  
CASE   V: 
A  

1.15         3.10 
3.15         5.25 

96.0       82.0 
93  7       81  4 

273       641         — 
233       614       427 

190 
261         370 

OK       OK 

OK         OK 
OK 

C  

1.40         4.35 

98.0       80  0 

117       507        — 

121          — 

OK         OK 

CASE.  VI: 
-A 

4  10         6  25 

95  3       82  4 

231        602 

050 

[Soft 
1  Warped 

C     . 

2  25         4  55 

96  2       82  0 

90       587 

150 

[  Cracked 
OK         OK 

D  

4.30         7  30 

94  6       79  2 

359       833        — 

259 

OK       OK 

(  Distorted 

CASE   VII: 
A  

3.35         6.5 

93.0       77.2 

343       gel         — 

243          — 

(  Soft 
OK         — 

C  

1.5           6.20 

93.7       76.4 

179       601         — 

154          — 

OK         OK 

CASE   VIII: 

A 

3  20         60 

95  2       81  2 

325       707       799 

315         436 

OK       OK 

OK 

C  

2.15         5.20 

92.5       75  5 

233       708        — 

267 

OK         OK 

CASE   IX: 
A  
B 

3.30         6.25 
1  55         40 

93.4       84.8 
94  9       79  § 

36S       677       817 
203        519       686 

383         446 
212         477 

OK       OK 
OK       OK 

OK          — 
OK          

CASE   X: 
A  

3.30         5  30 

94  2       83  8 

420       670       687 

363         428 

OK       OK 

OK         — 

B  

2.15         4.35 

95.1       80  4 

226       539       696 

220         483 

OK       OK 

OK         OK 

CASE   XI: 
B  
B     . 

2.20         4.45 
2  35         4  50 

95.0       80.0 
94  7       80  3 

208       507        — 
202       485       597 

218 
207         475 

OK       OK 

OK         OK 
OK         OK 

B  

2.15         5  10 

95  0       79  7 

213       495       603 

207        485 

OK       OK 

OK         OK 

CASE   XII: 
Mill  Report  .  . 
B 

3.0           5.58 
2  25         4  55 

—         79 
92  7       75  1 

290       627 
211        494       611 

224 
221         484 

—        OK 
OK       OK 

—          OK 
OK         OK 

B 

2  15          55 

93  1       77  0 

—        476       607 

215         484 

OK       OK 

OK         OK 

B 

2.10         5.25 

92  9       76  5 

207       493       612 

223        478 

OK       OK 

OK         OK 

A  
E  
CASE   XIII: 
Mill  Report  .  . 
A 

2.30         4.50 
2.43          5.08 

3.3            6.0 
2.50         4.30 

94.4       81.4 
95.0       77.9 

—         79.0 
95.2       82  5 

365       694       699 
421       802       846 

284       690        — 
429       748       710 

284         345 
325         389 

255          — 
302        343 

OK       OK 
OK       OK 

—        OK 
OK       OK 

OK         — 
OK         OK 

—         OK 
OK         — 

B  
B  

1.55         4.25 
2.15          5.0 

94.9       75.4 
94.0       77.0 

482        608 
195       482       609 

177         416 
225        479 

OK       OK 
OK       OK 

OK         OK 
OK         OK 

A 

2.30         4  50 

94  8       81  8 

325       691        696 

272         380 

OK       OK 

OK 

E           ... 

2.43          5.5 

96.5       77  5 

404       786       877 

306        417 

OK       OK 

OK         OK 

The  first  line  of  this  table  shows  the  requirements  of  the  American  Society  for  Testing  Materials  in  force 
when  this  was  printed,  in  December,  1909. 


MATERIALS  OF  CONSTRUCTION 


139 


TABLE   B 

TABLE  SHOWING  TIME  OF  SETTING  OF  SIXTEEN  SAMPLES  FROM  ONE  CAR  LOAD 


Bag 
No. 

Accelerated  Pat  Test 

Time 
Initial, 
bra.   min. 

of  set  . 
Final, 
hrs.  min. 

1 

Hard  sound                             

1       0 

3       5 

2 

Soft  left  glass  slightly  warped    

3      5 

4      0 

3 

Hard  sound                     

1     10 

3     20 

4 

Hard  sound                                   

0    45 

2     30 

5 

Hard  sound 

0    45 

2     45 

6 

Hard  sound                      

1      0 

3       5 

7 

Hard  broke  glass,  slightly  warped 

1       5 

2     50 

8 

Hard,  sound      

1     45 

3      0 

9 

Hard  sound                          

0    45 

2     50 

10 

Hard  left  glass 

0 

3     15 

11 

Hard  glass  broke  

0 

3      0 

12 

Hard  glass  cracked    . 

5 

3      0 

13 

Hard  sound 

10 

2     55 

14 
15 

Hard,  glass  cracked   
Hard  sound 

0 
1     50 

2     50 
2     35 

16 

Hard,  left  glass  

0     55 

2     45 

Tables  from  a  paper  in  the  Eng.  News,  Dec.  9,  1909,  by  Alexis  Saurbrey: 
"Comparison  of  Reports  on  Tests  of  the  Same  Cement  by  Various  Labo- 
ratories." 

of  simultaneous  tests  on  16  samples  taken  from  the  same  car, 
and  show  either  that  the  tester  was  incompetent,  which  is  hardly 
to  be  believed,  or  else  that  the  shipment  varied  very  considerably 
from  bag  to  bag,  which  of  course  was  emphatically  denied  by 
the  manufacturer. 

Undoubtedly,  there  is  a  vast  amount  of  dissatisfaction  with 
present  methods  of  cement  testing.  This  is  not  the  place  to 
discuss  whether  the  tests  now  in  common  use  are  too  difficult  to 
make  correctly,  or  the  present  staff  engaged  in  cement  testing  is 
deficient  in  skill,  or  both.  Professor  Waterbury  is  the  author  of 
the  following  statement: 

"  (1)  It  is  nearly  impossible  for  two  persons  to  obtain  the  same  numer- 
ical results  for  tests  upon  a  given  sample  of  cement.  (2)  The  results  ob- 
tained by  any  one  person,  who  has  had  some  experience  in  testing  cement, 
are  generally  in  accordance  with  other  results  obtained  by  the  same  observer. 
(3)  There  is  likely  to  be  a  greater  variation  in  the  results  of  the  24-hour  neat 


140  REINFORCED  CONCRETE  BUILDINGS 

tensile  tests  than  in  the  result  of  neat  tensile  tests  for  longer  periods  of  time. 
(4)  With  the  exception  of  the  24-hour  neat  tests,  there  is  likely  to  be  a 
greater  variation  in  the  results  of  1 :  3  mortar  tests  than  in  the  results  of 
neat  tensile  tests." 

Mr.  W.  P.  Taylor,  one  of  the  leading  cement  experts  in  this 
country,  when  addressing  the  National  Association  of  Cement 
Users,  said  in  part: 

"  Cement  testing  is  a  difficult  process  requiring  experience,  care,  pre- 
cision, and  knowledge,  and  hence  should  only  be  entrusted  to  well  qualified 
men,  but  too  often  this  important  work  is  relegated  to  utterly  untrained  and 
careless  operators  and  the  results  obtained  by  such  methods  are  really  worse 
than  nothing,  as  they  often  are  positively  misleading.  Many  tests  made 
at  the  present  time  by  supposedly  responsible  parties  are  ridiculous  in  their 
inaccuracy,  as  any  one  having  knowledge  of  this  subject  will  admit.  In- 
stances might  be  cited  without  number.  In  one  case  a  cement  was  rejected 
as  being  quick  setting,  but  an  investigation  showed  that  the  test  had  been  made 
in  a  hot  room  in  a  temperature  of  over  80°  F.  and  the  specimen  besides  placed 
directly  over  a  radiator  —  the  cement  itself  was  entirely  normal.  Strength 
tests  are  often  made  by  inexperienced  boys  committing  every  possible  error 
of  manipulation.  In  one  case  a  cement  reported  as  breaking  at  125  Ibs.  was 
found  to  give  a  strength  of  over  250  Ibs.  when  accurately  tested.  Cases  of 
unjust  rejection  on  the  accelerated  test  for  soundness  through  improper 
manipulation  and  interpretation  of  the  results  are  by  no  means  uncommon. 
Of  the  sieves  used  for  testing  fineness,  not  one  in  four  has  been  properly 
standardized.  These  inaccuracies,  it  must  be  remembered,  are  not  only 
found  in  the  small  laboratories,  but  only  too  often  in  those  of  some  reputa- 
tion, and  the  cause  may  be  found  to  be  entirely  due  to  the  desire  to  cheapen 
the  cost. 

It  should  be  recognized  at  once  that  if  cement  tests  are  made  it  is  worth 
while  to  make  them  well,  even  at  possibly  a  somewhat  increased  expense."  l 

Granting,  however,  that  satisfactory  and  reliable  cement  has 
been  received,  it  becomes  necessary  to  so  store  and  use  the 
cement  that  it  will  not  deteriorate.  For  this  reason  cement 
must  be  stored  in  a  house  of  substantial  design,  where  water  or 
even  dampness  will  not  penetrate.  The  temperature  must  be 
kept  as  low  as  possible  in  the  summer,  as  a  temperature  of  from 
80°  to  100°  may  seriously  interfere  with  the  setting  qualities  of 
the  cement,  changing  normal  cement  into  extremely  quick  setting 
cement.  This  knowledge  should  always  be  imparted  to  every  one 
on  the  job,  so  that  close  watch  is  kept  of  all  batches  deposited 
in  the  forms.  If  the  concrete  hardens  in  the  wheelbarrows,  it 
must  not  be  used;  it  is  playing  with  fate  to  retemper  such  con- 
Quoted  from  an  editorial  in  the  Engineering  News,  Dec.  9,  1909. 


MATERIALS  OF  CONSTRUCTION  141 

crete  with  more  water,  as  it  hardens  very  slowly  and  probably 
never  reaches  the  calculated  strength.  Without  doubt,  many 
accidents  may  be  traced  back  to  neglect  of  this  one  point. 

On  a  certain  large  foundation  job  this  very  thing  happened. 
When  the  pier  forms  were  removed  the  concrete  was  quite  wet 
and  soft,  and  fell  entirely  apart.  Samples  were  taken,  and  owing 
to  the  very  plastic  condition  of  the  concrete  it  was  possible  to 
mold  test  cubes  which  were  allowed  to  harden.  As  expected, 
the  7  and  28  day  specimens  had  barely  cohesion  enough  to 
stand  the  handling  of  placing  them  in  the  machine.  One  wall, 
16  inches  thick  and  4  feet  high,  was  allowed  to  remain  in  place, 
as  it  carried  no  loads.  When  six  weeks  old  it  was  still  so  soft 
that  impressions  were  readily  made  with  the  thumb  alone,  but 
in  the  course  of  a  few  months  the  concrete  seemed  to  get  quite 
hard,  and  it  was  decided  to  leave  the  wall  in  place.  At  the 
present  time,  the  wall  is  about  five  years  old  and  apparently 
in  a  satisfactory  condition.  Similar  cases  have  come  under 
observation  from  time  to  time,  so  that  the  conditions  just 
described  are  by  no  means  exceptional. 

Dampness  is  best  avoided  by  careful  attention  to  ventilation 
on  clear  and  dry  days.  While  opinions  are  divided  on  this 
subject,  it  seems  the  best,  and  certainly  the  safest,  practice  to 
reject  cement  with  lumps  or  cakes. 

Under  any  circumstances,  when  cement  is  received  it  should 
be  well  seasoned  and  ready  for  immediate  use,  and  it  should  be 
used  at  once.  Hence,  "  warehouse  cement  "  must  always  be 
regarded  with  suspicion.  On  large  work,  the  most  careful  cement 
inspection  and  the  most  scientific  testing  may  easily  be  had  at 
a  negligible  cost,  but  on  very  small  work  the  problem  becomes 
quite  difficult.  The  parties  most  likely  to  suffer  are  the  small 
manufacturer  of  cement  blocks  and  the  sidewalk  man. 

The  cement  used  in  reinforced  concrete  work  is  always  Port- 
land Cement;  the  exceptions  are  so  few  that  they  may  be  said 
not  to  exist.  Each  car  load  received  is  sampled  individually, 
a  small  amount  from  each  bag  out  of  every  30  bags  received, 
or,  in  case  of  delivery  in  barrels,  1  barrel  out  of  every  10  is 
customarily  sampled.  These  samples  are  mixed  to  an  average 
sample  representing  the  car  load,  taken  to  the  testing  laboratory, 
and  there  submitted  to  the  standard  tests.  In  order  that  the 
average  sample  may  fairly  represent  the  car  load,  the  individual 


142  REINFORCED  CONCRETE  BUILDINGS 

samples  from  the  several  bags  or  barrels  must  be  taken  from 
various  parts  of  the  car.  The  tester  marks  each  and  every 
package  with  the  name  of  the  testing  laboratory  and  the  number 
of  the  test,  each  car  load  receiving  its  own  "  test  number." 
The  aggregate  amount  of  cement  taken  out  of  each  car  load  is 
about  16  Ibs.,  one-half  of  which  is  sent  to  the  laboratory,  and  the 
other  half  is  stored  for  reference  by  the  engineer.  Eight  pounds 
are  usually  sufficient  for  the  purpose  of  the  tests  in  common  use. 

On  work  of  any  importance  it  is  good  practice  to  make  field 
tests  of  the  cement  (setting  time  and  soundness  principally)  to 
guard  against  changes,  and  also  compression  tests  on  cubes 
made  from  concrete  taken  from  the  mixer  or  the  wheelbarrows. 
These  tests  simply  supplement  the  laboratory  tests  and  cannot 
replace  them;  however,  the  compression  test  on  the  concrete  as 
actually  used  is  in  itself  a  very  excellent  check  on  the  efficiency 
of  the  mixture  used,  and  gives  also  important  information  as 
to  the  proper  time  for  removal  of  the  forms.  As  a  matter  of 
fact,  the  engineer  is  not  greatly  interested  in  the  strength  of 
cement  as  tested  in  the  laboratory  under  conditions  which  never 
obtain  in  the  field,  and  he  is  probably  relying  upon  the  compli- 
cated and  difficult  laboratory  test  because  nothing  better  is 
available.  The  compression  test  in  the  field  might  well  be  used 
more  extensively,  although,  as  far  as  rejection  or  acceptance  of 
a  given  cement  shipment  is  concerned,  it  is  of  no  importance 
whatever. 

The  consulting  engineer  is  sometimes  called  upon  to  examine 
an  existing  structure,  and  inasmuch  as  such  examination 
usually  has  its  cause  in  troubles  of  various  kinds,  he  might  be 
interested  in  knowing  what  the  original  proportions  of  the 
concrete  were.  Unfortunately,  it  seems  that  there  is  no  very 
satisfactory  method  whereby  such  information  can  be  obtained, 
and  if  obtainable,  testimony  along  these  lines  would  probably 
have  little  weight  in  court  unless  a  large  number  of  samples  were 
analyzed.1  This  is  also  true  of  compression  tests  made  on 

1  See  also  paper  in  Eng.  News,  1908,  p.  46,  by  Prof.  R.  L.  Walls,  who  made 
a  successful  analysis  of  this  kind. 

One  case  of  this  kind  happened  in  Oakland,  Cal.,  where  skimping  of 
the  cement  was  proved  to  the  satisfaction  of  the  jury  by  careful  chemical 
analysis,  based  upon  the  amount  of  lime  in  the  analyzed  concrete.  Many 
years  afterward,  I  came  across  evidence  of  other  nature  that  showed  the 
chemical  analysis  to  be  correct.  —  ERNEST  L.  RANSOME. 


MATERIALS  OF  CONSTRUCTION  143 

cubes  or  cylinders  cut  from  the  concrete,  because  it  would  be 
difficult,  but  not  impossible,  to  show  that  the  pieces  were  not 
injured  in  the  process  of  cutting. 

Perhaps  it  is  well  here  to  call  attention  to  the  fact  that  the 
size  and  shape  of  the  test  specimen  has  some  influence  upon  its 
strength,  so  that  results  obtained  from  4"  and  6"  cubes  cannot 
be  directly  compared  without  reference  to  the  laws  governing 
such  cases. 

In  tests  on  concrete  and  mortar,  the  relative  size  of  the  aggre- 
gate and  the  test  specimen  might  also  have  some  influence. 

Sand.  Next  to  the  cement,  the  sand  is  the  important  factor 
in  determining  the  strength  of  the  concrete.  Various  elab- 
orate theories  exist  whereby  the  proper  composition  of  the  sand 
may  be  determined;  where  a  large  concrete  job  is  to  be  sup- 
plied from  a  uniform,  local  supply  of  large  capacity  such  inves- 
tigations may,  of  course,  be  of  great  value,  as  it  is  possible  to 
determine  just  what  should  be  added  or  deducted  in  order  to 
get  the  best  results.  But  ordinarily  such  investigations  are  of 
little  value,  as  the  character  of  the  sand  may  vary  from  day  to 
day,  if  indeed  it  does  not  vary  from  shovelful  to  shovelful. 
Hence  a  quick  and  cheap  test  is  required  for  daily  or  occasional 
use,  and  such  a  test  we  have.  It  consists  in  simply  comparing 
the  strength  of  briquettes  made  from  "  standard  "  sand  and 
cement  with  that  of  similar  briquettes  made  at  the  same  time 
from  the  proposed  sand  and  the  same  cement.  Obviously,  this 
method  of  testing  is  free  from  nearly  all  the  objections  made 
against  the  usual  cement  test,  as  only  comparison  is  wanted  and 
not  absolute  figures.  The  standard  sand  is  not  particularly 
strong  sand  if  used  for  building  purposes,  so  that  for  good 
results  the  proposed  sand  should  give  a  tensile  strength  25  to  50 
per  cent,  in  excess  of  that  obtained  with  standard  sand;  when 
the  strength  is  about  equal,  the  sand  may  be  termed  "  pass- 
able "  if  only  low  stresses  are  used  in  the  design,  while  sand 
well  below  standard  may  be  rejected  without  error.  Speci- 
fications drawn  along  this  line  avoid  entirely  all  questionable 
and  unfair  regulations.  Where  water-tight  work  is  required, 
the  more  elaborate  granularmetric  analysis  may  be  used  if 
the  supply  is  uniform  in  character.  Frequently,  an  addition 
of  a  small  amount  of  fine  sand,  or  preferably  stone  dust,  greatly 
improves  the  strength  of  the  concrete. 


144  REINFORCED  CONCRETE  BUILDINGS 

It  is  well  understood  by  skilled  concrete  men  that  the  best 
grade  of  sand  is  clean,  sharp,  and  well  graded  from  fine  to 
coarse,  and  these  words  are  therefore  usually  inserted  in  the 
specifications.  It  is  believed  that  only  in  exceptional  cases 
such  specifications  are  enforced,  and  the  policy  of  writing  speci- 
fications which  nobody  can  or  will  enforce  is  not  to  be  recom- 
mended for  obvious  reasons.  Up  to  three  per  cent,  of  impurities 
are  not  usually  injurious,  but  sometimes  even  a  much  smaller 
quantity  of  clay  is  detrimental,  especially  if  the  several  grains 
are  covered  with  a  thin  film  of  clay.  The  test  recommended 
above  settles  such  questions  at  once,  provided  that  the  mix- 
ture made  in  the  laboratory  is  not  such  that  the  film  is  removed; 
too  much  mixing  is  as  bad  as  not  enough.  On  the  job,  however, 
the  mixing  must  of  course  be  greatly  prolonged  if  the  grains 
are  coated  so  as  to  wash  the  film  off  the  sand. 

The  specifications  must  state  the  maximum  size  of  grain 
allowed;  usually  the  sand  is  required  to  pass  a  screen  with  four 
meshes  per  lineal  inch. 

Stone.  The  stone  should  be  clean  and  hard,  two  require- 
ments easily  complied  with.  Loose  dust  should  not  be  allowed 
when  the  concrete  mixture  is  specified  in  proportions  as  1:2:4 
or  similar  ratios,  because  the  dust  acts  as  so  much  sand  and 
decreases  the  strength  of  the  mortar  base.  Some  dust  always 
clings  to  the  stone,  hence  the  word  "  loose  "  should  be  used. 
On  the  other  side,  if  it  should  be  found  desirable  to  use  "  run 
of  crusher  "  with  some  additional  sand  there  is  no  reason  why 
good  results  cannot  be  obtained  in  that  way  with  continuous 
and  intelligent  supervision.  As  a  general  thing,  the  engineer  will 
save  himself  a  large  amount  of  trouble  by  insisting  upon  separate 
stock  piles  for  sand  and  stone,  and  specify  his  materials  by 
definite  proportions.  There  can  then  be  no  room  for  dispute. 

If  the  specification  suggested  above  is  used,  the  stone  should 
be  required  to  pass  a  ring  f"  in  diameter  for  thin  reinforced 
concrete  pieces  as  used  for  floors,  beams,  and  columns;  for  heavy 
work,  the  size  may  go  up  to  2"  ring  or  larger.  The  stone  should 
be  retained  on  the  \"  mesh  screen,  perhaps  with  a  small  allow- 
ance, so  that,  for  instance,  5  or  10  per  cent,  may  pass  through 
the  screen,  the  balance  to  be  retained.  Certain  kinds  of  rock 
give  oblong  stones,  and  a  maximum  length  should  be  specified, 
for  instance  1". 


MATERIALS  OF  CONSTRUCTION  145 

Attention  is  called  to  the  ever-increasing  use  of  furnace 
slag,  a  by-product  from  the  manufacture  of  pig-iron.  Slag  makes 
excellent  concrete  if  used  with  the  proper  proportions  of  mor- 
tar, for  instance,  1:2:3  or  1 :  2  :  3J.  The  slag  is  very  dry  and 
absorbs  water  in  large  quantities;  the  stock  pile  should  there- 
fore be  kept  soaking  wet  at  all  times.  Otherwise  the  concrete 
may  not  set  up  well. 

Boiler  cinders  should  not  be  allowed  in  reinforced  concrete 
work,  as  little  as  soft  limestone  or  soft  sandstone,  or  any  kind 
of  stone  disintegrating  under  the  influence  of  the  atmosphere. 
Fair  concrete  may  be  made  from  soft  or  friable  aggregate  by 
limiting  the  time  of  mixing  to  a  minimum;  good  limestone  makes 
excellent  and  very  hard  concrete,  and  crushed  brick  if  not  very 
soft  makes  a  very  fair  concrete  for  many  purposes.  Brick 
dust  is  a  good  substitute  for  natural  sand. 

Certain  kinds  of  shale  have  great  toughness  when  in  the 
natural  deposit,  but  fall  to  a  powder  when  exposed  to  the  influ- 
ence of  the  air.  Conglomerate  cemented  together  from  a  large 
number  of  small  pieces  must  be  prohibited,  even  if  apparently 
hard.  The  authors  recall  one  or  two  instances  where  this  kind 
of  stone  led  to  very  serious  trouble.  Certain  kinds  of  slag  con- 
tain very  injurious  chemicals,  such  as  sulphate  of  lime,  etc. 
Chemical  analysis  should  be  insisted  upon  before  the  use  of  an 
unknown  slag. 

Steel.  The  selection  of  the  steel  is  rather  embarrassing, 
each  "  system  "  claiming  special  advantages  of  its  own.  In 
most  cases  these  advantages  exist  largely  on  paper  only,  the 
fact  being  that  almost  any  kind  of  steel  may  be  used  with  suc- 
cess. The  form  of  the  particular  bar  proposed  is  a  compara- 
tively small  matter;  the  real  importance  is  in  the  material  from 
which  the  bar  is  manufactured,  and  the  method  of  manufac- 
ture. In  their  own  work,  the  authors  prefer  the  cold  twisted 
square  bar.  The  specifications  should  call  for  minimum 
ultimate  strength,  minimum  limit  of  elasticity,  minimum  per- 
centage of  elongation,  and  a  bending  test.  The  engineer  is 
interested  in  the  kind  of  steel  furnished,  not  in  the  method  of 
manufacture. 

Plain  Bars.  Round,  square,  or  flat  bars  are  used,  but  the 
round  bar  should  be  favored  as  easier  to  handle,  and  flat  bars 
are  generally  considered  as  giving  smaller  bonding  strength  hi 


146  REINFORCED  CONCRETE  BUILDINGS 

the  concrete.  High  tension  or  low  tension  bars  may  be  used  if 
a  proper  length  of  anchoring  is  had  in  each  case.  The  high 
tension  bars  are  frequently  rerolled  from  old  railroad  rails  — 
in  itself  a  very  good  idea.  But  this  steel  is  rather  high  in 
carbon  and  requires  extra  care  in  manufacture;  rerolling  at 
too  high  or  too  low  temperatures  may  be  the  cause  of  brittle- 
ness  and  other  trouble.  Many  engineers  decline  to  use  rerolled 
or  hot  twisted  bars  for  this  reason,  and  it  cannot  be  denied  that 
unless  properly  tested,  such  steel  may  not  be  what  is  expected 
and  required.  Figure  130  shows  a  bad  piece  of  rerolled  steel. 


FIGURE  130.     UPPER  BAR:  REROLLED  STEEL,  IMPROPERLY  MANU- 
FACTURED.      LOWER  BAR:  GOOD  SPECIMEN. 

Both  bars  from  the  collapse  of  the  Henke  Building  (Column  Rods). 
Photo  by  Alexis  Saurbrey,  who  examined  ruins  for  the  owner. 

Deformed  Bars.  The  great  class  of  bars  distinguished  by 
projections  and  recesses  on  the  surfaces  have  this  in  common, 
that  if  sufficient  concrete  surrounds  the  bar,  it  is  harder  to  pull 
out  than  the  plain  bar  of  same  cross-section.  It  is  almost 
impossible  to  discriminate  between  all  these  bars  which  belong 
to  types  merging  one  in  the  other  by  gradual  steps. 

Patent  royalties  are  collected  on  most  of  these  bars;  the 
twisted  bars,  both  hot  and  cold  twisted,  are  exceptions.  The 
value  of  the  steel  as  reinforcement  is  not  greatly  affected  by 


MATERIALS  OF  CONSTRUCTION  147 

the  various  forms  of  projections  in  use;  bars  with  deep  grooves 
should  not  be  used,  as  water  instead  of  concrete  is  likely  to 
collect  in  the  pockets,  especially  on  the  under  side  of  the  bar. 
If  deformed  bars  are  used  it  should  be  of  a  type  having  the  same 
cross-sectional  area  at  all  points  of  the  length. 

Wire  Mesh.  Reinforcement  with  ready-made  wire  mesh 
is  adapted  only  for  short  span  slabs,  and  even  then  additional 
bars  of  larger  diameter  are  often  used.  The  cost  is  high,  and 
attempts  are  therefore  frequently  made  of  talking  the  buyer 
into  allowing  much  higher  stresses  on  wire  fabrics  than  on  plain 
steel.  It  is  of  course  possible  that  our  present  methods  of  cal- 
culating stresses  in  slabs  are  in  error,  but  the  proof  has  yet  to 
be  furnished.  In  the  meantime  we  cannot  consistently  allow 
stresses  on  drawn  wire  as  high  as  30,000  or  40,000  Ibs. /square 
inch,  even  if  this  material  has  a  tensile  strength  of  100,000  to 
120,000  Ibs. /square  inch. 

Requirements  and  Tests  for  Steel.  The  ultimate  strength 
of  bars  used  for  reinforcing  purposes  should  be  at  leas't  four 
times  the  allowable  stress,  and  the  elastic  limit  should  be  at 
least  twice  the  allowable  stress.  Of  these  two,  the  latter  is 
by  far  the  most  important;  a  high  elastic  limit  increases  the 
factor  of  safety  of  the  entire  structure,  although  not  in  direct 
ratio.  Both  of  these  figures  are  easily  determined  by  a  ten- 
sile test,  except  in  the  case  of  bars  without  a  definite  elastic 
limit,  such  as  cold  twisted  bars.  In  this  case,  the  strength  and 
elastic  limit  may  be  determined  either  before  or  after  twisting; 
generally  speaking,  the  ultimate  strength  is  raised  about  33 
per  cent,  by  twisting  soft  stock,  while  the  elastic  limit,  if  it 
can  be  at  all  determined,  will  be  found  to  be  about  75  per  cent, 
of  the  ultimate  strength.  This  applies  to  cold  twisted  bars 
only;  hot  twisting  does  not  change  the  strength  or  elastic 
properties  of  the  bar. 

The  bending  test  is  very  important,  as  practically  all  the 
bars  are  bent  on  the  job;  all  bending  should  of  course  be  done 
cold.  For  high-carbon  steel  it  is  usually  specified  that  the  bar 
must  bend  cold  around  a  pin  four  times  the  diameter  of  the 
bar  without  showing  signs  of  distress.  Good  cold  twisted  bars 
will  easily  bend  around  a  pin  twice  the  diameter  of  the  bar. 
Three  or  four  diameters  are  however  more  commonly  spe- 
cified. Soft  stock  should  fold  flat  upon  itself  without  showing 


148  REINFORCED  CONCRETE  BUILDINGS 

signs  of  cracking  or  checking.  It  follows  that  the  kind  of 
steel  to  specify  may  depend  largely  upon  the  bending  problems 
encountered. 

The  elongation  serves  practically  the  same  purpose  as  the 
bending  test.  The  minimum  amount  specified  varies  from 
10  per  cent,  for  high-tension  steel  to  20  per  cent,  or  25  per  cent, 
for  soft  steel.  These  figures  must  not  lead  us  to  believe  that 
the  smaller  per  cent,  of  elongation  at  fracture  is  a  point  in 
favor  of  the  high-tension  steel;  in  fact,  the  coefficient  of  elasticity 
is  practically  the  same  for  all  kinds  of  steel  in  common  use,  and 
the  elongation  under  a  working  load  depends  upon  the  coefficient 
of  elasticity. 

The  following  quotation  from  the  Cleveland  Building  Code 
may  be  of  interest: 

"Steel  reinforcement  shall  be  divided  into  two  classes,  Medium  and 
High  Tension.  Medium  steel  shall  have  an  ultimate  strength  of  60,000  to 
70,000  Ibs. /square  inch,  and  shall  conform  to  the  Manufacturers'  Standard 
Specifications  as  revised  Feb.  6,  1903.  High-tension  steel  shall  have  an  ulti- 
mate strength  of  not  less  than  80,000  Ibs. /square  inch,  and  an  elastic  limit 
of  not  less  than  45,000  Ibs. /square  inch.  The  elongation  shall  be  at  least 
ten  per  cent,  in  eight  inches.  Bars  shall  bend  cold  around  a  pin  of  diameter 
equal  to  4  times  the  least  dimension  of  the  bar  without  showing  signs  of 
cracking." 

Tiles.  The  tiles  are  usually  made  12"X12"  in  plan;  the 
width  usually  cannot  be  changed,  but  tiles  12  "  X  6"  or  12  "  X  18 " 
may  sometimes  be  obtained.  The  thickness  of  walls  and  webs 
is  frequently  about  \" ,  subject  to  variation.  Dense  or  semi- 
porous  tiles  can  be  obtained;  it  makes  little  difference  in 
the  results  which  is  selected.  The  surface  should  be  deeply 
scored  so  that  the  plaster  may  be  firmly  bound  to  the  tile; 
only  in  the  roughest  kind  of  work  are  the  tiles  left  uncovered 
on  the  exposed  bottom  side.  In  burning,  the  tiles  shrink;  a 
well-burned  tile  is  often  as  much  as  f  "  smaller  each  way  than 
specified.  This  should  be  made  up  in  concrete,  so  that  the 
plans  should  show  the  full  thickness  of  floor,  not  the  thickness 
of  concrete  topping.  At  the  same  time,  tiles  may  be  too  large, 
in  which  case  the  minimum  amount  of  concrete  to  be  placed 
over  the  top  of  the  tiles  should  be  specified.  Broken,  badly 
warped,  or  otherwise  defective  tiles  should  not  be  allowed. 
Before  the  concrete  is  run,  the  tiles  should  be  made  soaking  wet, 
as  they  will  otherwise  absorb  the  moisture  from  the  concrete. 


MATERIALS  OF  CONSTRUCTION  149 

Concrete.  Hand-mixing  is  used  only  in  exceptional  cases; 
the  engineer  should  reserve  the  right  to  permit  hand-mixing  if 
practically  unavoidable.  Machine-mixing  on  continuous  mixers 
is  not  desirable;  the  mixer  should  preferably  be  a  revolving 
batch-mixer  of  approved  design.  The  aggregate  and  cement 
is  measured  by  volume;  it  is  very  convenient  to  take  the  cubic 
foot  as  unit  and  consider  the  bag  as  containing  one  cubic  foot 
of  loose  cement.  When  the  job  is  started  the  wheelbarrows 
or  receiving  bins  are  checked  by  means  of  the  standard  unit, 
and  the  required  depth  of  filling  marked.  All  wheelbarrows 
and  bins  must  be  brought  to  a  level  when  filled;  a  small  top  on 
the  wheelbarrow  load  looks  like  a  small  matter,  but  may  in 
fact  mean  a  material  decrease  in  the  proportional  amount  of 
cement.  Wheelbarrows  containing  the  required  amount  when 
level  full  can  easily  be  obtained. 

Sufficient  water  should  be  added  so  as  to  make  the  mixed 
concrete  into  a  flowing  paste  that  will  pour  from  the  wheel- 
barrow. The  concrete  is  mixed  with  an  excess  of  water  if  pools 
are  immediately  formed  on  top  of  the  concrete  when  deposited 
in  the  forms:  the  pools  increase  in  size,  the  water  finds  an  out- 
let to  a  lower  point,  and  the  cement  is  washed  away  from  the 
mortar.  Inclined  "  sand-streaks  "  on  the  sides  of  girders  or 
beams  are  usually  due  to  this  cause.  Years  ago  "  dry  "  concrete 
was  specified,  but  the  manipulation  becomes  so  difficult  with 
dry  concrete  that  "  wet  "  concrete  soon  became  universally 
used,  and  at  present  there  is  a  tendency  to  exaggerate  the 
amount  of  water.  Where  the  concreting  of  large  girders  pro- 
ceeds from  one  end,  and  the  working  face  of  the  concrete  body 
is  sloping,  the  excess  of  water  naturally  seeks  the  lower  level 
in  the  part  of  the  girder  box  not  yet  concreted.  The  water 
carries  "  laitance  "  with  it,  and  this  sets  without  hardening, 
forming  a  white  plaster-like  film  on  the  bottom  of  the  girder, 
often  |"  thick  or  more.  Such  conditions  should  of  course  be 
avoided. 

The  mixing  must  continue  until  all  parts  are  thoroughly 
incorporated  in  the  mixture  and  all  stones  covered  with  mortar: 
the  concrete  will  then  be  of  uniform  consistency  and  color,  and 
if  sufficient  water  only  is  used  there  will  be  no  precipitation  in 
the  bin  or  in  the  wheelbarrows  of  the  heavier  particles.  This 
separation  is  much  more  likely  to  take  place  if  perfectly  clean 


150  REINFORCED  CONCRETE  BUILDINGS 

sand  is  used,  especially  if  the  grains  are  round,  and  lake  or 
river  sand  does  therefore  require  less  water  and  more  care  than 
bank  sand  containing  a  slight  amount  of  clay.  As  the  aggre- 
gate contains  varying  amounts  of  moisture  in  the  different  parts 
of  the  stock-pile,  and  as  the  stock  pile  seldom  is  perfectly  uni- 
form throughout,  no  hard  and  fast  rule  can  be  laid  down  for 
the  requisite  amount  of  water.  The  amount  of  water  used, 
and  the  number  of  turns  given  the  mixer  before  dumping  should 
be  left  to  a  competent  man  in  charge  of  the  mixer.  The  mixer- 
man  is  an  important  person  on  the  job. 

The  concrete  must  be  deposited  in  the  forms  before  it  stif- 
fens perceptibly;  concrete  held  longer  than  that  time  in  either 
the  receiving  hopper  or  the  wheelbarrows  should  be  wasted. 
The  concrete  must  be  well  spaded  and  churned  in  the  forms 
to  insure  dense  concrete  and  fine  surfaces.  The  entire  con- 
crete work  should  be  under  the  personal  and  direct  supervision 
of  an  intelligent  and  careful  man;  this  man  should  be  present 
at  all  times  when  concreting  is  going  on,  and  should  have  no 
other  duties  during  concreting.  If  the  engineer  or  owner  de- 
sires to  have  an  inspector  of  his  own  on  the  job  at  the  same 
time,  so  much  better,  but  it  should  be  made  clear  to  the  con- 
tractor and  his  men  that  such  inspection  is  for  the  purpose  of 
enforcing  good  work  only,  not  for  the  purpose  of  waiving  the 
specifications.  The  concrete  must  be  conveyed  to  and  depos- 
ited in  the  forms  in  such  a  manner  that  the  steel  reinforcement 
is  not  disturbed  and  so  that  older  concrete  is  not  injured.  The 
mechanical  plant  must  not  be  braced  to  the  form  work  or  the 
building,  or  have  any  solid  connection  therewith.  All  these 
rules  are  dictated  by  common  sense,  but  nevertheless  daily  vio- 
lated, and  it  is  therefore  well  to  put  them  in  the  specifications. 

Joints  in  the  concrete  work  should  not  be  allowed  except 
at  the  natural  end  of  each  day's  run;  it  is  therefore  essential 
that  the  plant  be  in  such  shape  that  breakdowns  are  avoided. 
The  necessary  joints  must  be  well-defined,  straight  lines,  prefer- 
ably through  the  center  of  the  span  of  all  slabs  and  beams. 
The  joint  should  be  made  perpendicular,  not  sloping;  the  ver- 
tical joint  is  easily  repaired  in  case  of  trouble.  New  concrete 
should  be  joined  to  old  concrete  only  after  the  surface  of  the 
old  concrete  has  been  removed  by  mechanical  or  chemical 
means,  and  the  rough  surface  made  in  this  way  thoroughly 


MATERIALS  OF  CONSTRUCTION  151 

cleaned  with  scrubbing-brushes  and  water.  Neat  cement 
paste  is  then  rubbed  into  the  clean  surface,  and  concreting 
proceeded  with  at  once.  The  care  taken  is  of  no  avail  if  the 
cement  paste  is  allowed  to  dry  out  or  set  before  the  new  con- 
crete is  put  on.  The  slab  and  the  beam  supporting  it  are  usu- 
ally run  in  one  continuous  operation,  without  any  joint  between 
them;  but  the  design  can  easily  be  so  arranged  that  a  joint  can 
be  made  if  desired. 

The  concrete  gang,  and  especially  its  foreman,  should  be 
made  to  understand  that  it  is  their  duty  to  make  concrete  which 
will  not  require  after  treatment  to  make  up  for  their  careless- 
ness or  haste.  No  pointing-up  should  be  allowed  before  the 
engineer  has  seen  and  approved  the  concrete;  but  when  so 
directed  the  contractor  must  at  once  proceed  with  the  pointing. 

In  monolithic  construction,  the  columns  should  be  run  a 
sufficient  length  of  time  ahead  of  the  floor,  to  allow  the  con- 
crete in  the  columns  to  settle  and  shrink;  the  interval  may 
conveniently  be  utilized  in  putting  the  floor-steel  in  place. 

The  setting  of  the  concrete  is  greatly  influenced  by  atmos- 
pheric conditions.  Hot  weather  accelerates  the  action,  and 
cold  weather  retards  it.  Otherwise,  neither  heat  nor  cold  need 
have  any  injurious  action  on  the  concrete  if  proper  precautions 
are  taken.  In  hot  weather,  it  may  be  necessary  to  cover  the 
green  concrete  against  the  direct  rays  of  the  sun,  and  in  any 
case  the  concrete  should  be  sprinkled  liberally  to  make  up  for 
the  loss  of  water  by  evaporation,  as  concrete  cannot  gain  its 
full  strength  without  water.  Much  more  serious  is  the  action 
of  frost,  and  especially  of  repeated  freezing  and  thawing;  the 
precautions  to  be  taken  in  the  summer  are  simple  and  cheap 
compared  with  those  required  in  winter,  where  the  weather 
may  suddenly  change  from  mild  to  bitter  cold.  The  concrete 
is  made  with  heated  materials  and  heated  water;  the  green 
concrete  is  covered  with  a  tent  or  boards  with  straw  on  top, 
but  not  manure,  which  is  said  to  injure  the  concrete;  in  fact, 
one  or  perhaps  two  accidents  have  been  ascribed  to  the  use  of 
manure.  Moist  heat  is  supplied  to  the  space  below  the  green 
concrete,  and  between  the  concrete  and  the  covering. 


CHAPTER   XI 

FLOOR  SYSTEMS 

BROADLY  speaking,  reinforced  concrete  may  be  used  in  one 
of  two  ways :  cast  in  place,  or  cast  in  the  yard  and  erected  after- 
wards when  hard.  In  the  first  case,  the  construction  becomes 
more  or  less  continuous  by  virtue  of  the  method  of  erection, 
and  the  continuity  is  then  usually  emphasized  in  the  design, 
so  that  all  parts  are  thoroughly  tied  and  united  together.  This 
type  of  construction  is  therefore  referred  to  as  "  Monolithic." 
In  the  second  case,  the  structure  is  divided  into  separate  pieces 
or  "  units,"  and  both  the  designer  and  the  erector  must  then  so 
articulate  the  building  that  the  weights  and  dimensions  of  the 
several  pieces  come  within  reasonable  limits.  In  distinction 
from  the  monolithic  work,  this  type  of  construction  is  referred 
to  as  "  Unit."  A  large  number  of  "  systems  "  exist  within 
either  of  these  broad  divisions,  several  of  which  claim  protec- 
tion under  United  States  patents,  and  all  of  which  claim  either 
superior  strength  or  greater  economy.  It  would,  however,  be 
outside  the  scope  of  this  book  to  enter  into  a  discussion  of 
these  points;  moreover,  there  is  no  accepted  standard  of  com- 
parison, so  that,  in  all  probability,  the  contractor's  bid  or 
proposal  on  any  given  building  gives  the  only  safe  way  of 
determining  the  relative  cost  in  each  case. 

Some  types  of  construction  are  essentially  monolithic,  such 
as  the  flat-plate  and  column  construction,  and  the  tile  con- 
crete construction.  Others  are  essentially  of  the  unit  type, 
such  as  the  Visintini  System,  where  each  unit  is  a  small  truss 
in  itself.  The  common  "  ribbed  floor,"  with  beams  and  gir- 
ders supporting  the  floor  plate  proper,  is  usually  built  in  a 
strictly  monolithic  way,  but  recently  considerable  efforts  have 
been  made  toward  perfecting  the  "  unit  "  method  for  floors  of 
this  kind.  Two  distinct  methods  have  been  followed  in  the 
unit  construction:  (1)  Each  beam  or  girder  is  a  complete  carry- 

152 


FLOOR  SYSTEMS  153 

ing  member  in  itself,  and  in  that  case  the  slab  portion  is  usually 
molded  on  the  ground,  and  set  in  place  when  hard.  (2)  The 
beams  and  girders  are  not  complete  carrying  members,  the 
floor  slab  proper  forming  the  upper  flange  of  the  T-beam,  and  in 
this  case  the  beams,  girders,  and  columns  are  cast  on  the  ground 
and  set  in  place  when  hard,  while  the  slab  proper  is  cast  in  place 
over  the  top  of  the  beams,  and  serves  the  dual  purpose  of  tying 
the  building  together,  and  of  forming  the  compression  flange 
of  the  beams  and  girders.1 

Design.  After  the  mathematical  design  has  been  perfected, 
additional  steel  must  be  introduced  to  take  care  of  shrinkage 
stresses,  especially  in  the  slabs.  These  bars  are  disposed  cross- 
wise over  the  tension  rods,  and  also  serve  as  "distributing  rods11; 
the  structures  erected  under  the  so-called  "Monier  System" 
always  had  large  quantities  of  such  bars  which  greatly  strengthen 
the  building  as  a  whole.  The  exterior  belt  courses  should  have 
ample  additional  reinforcement,  and  these  bars  should  run  con- 
tinuously around  the  entire  building  at  each  floor,  with  sufficient 
lap  at  each  joint.  In  the  " tile-concrete "  construction,  the 
value  of  such  additional  steel  is  too  frequently  overlooked, 
although  it  is  here  of  particular  importance  owing  to  the  absence 
of  secondary  beams.  It  seems  to  be  an  open  question  to  what 
extent  the  slab  can  be  considered  as  active  in  compression,  and 
the  laws  governing  the  influence  of  the  width  of  the  top  flange 
are  practically  unknown  except  for  a  few  sporadic  tests.  It  can, 
however,  be  stated  that  the  active  width  of  slab  depends  upon 
the  thickness  of  the  slab,  and  upon  the  intensity  of  compres- 
sion stress,  and  the  stiffness  of  the  system  as  a  totality  probably 
enters  to  some  degree.  Common  rules  are:  One-third  or  one- 
sixth  of  the  span;  two-thirds  of  the  distance  between  beams; 
six  or  ten  times  the  width  of  the  stem,  etc.  None  of  these  rules 
is  derived  from  either  test  or  convincing  analysis. 

In  monolithic  construction,  the  questions  of  bearing  for 
beam  —  or  girder  —  ends  do  not  usually  arise,  and  when  they 
do,  the  strength  of  the  supporting  material  is  usually  the  govern- 
ing factor.  In  many  brick  buildings  the  pressure  on  the  wall 
bearing  has  been  limited  to  200  or  250  Ibs.  per  square  inch. 

The  connections  between  the  several  elementary  parts  are 

1  This  type  of  construction  is  covered  by  my  U.  S.  Patents  of  March  4, 
1902,  No.  694,577,  and  April  20,  1909,  No.  918,699.  — ERNEST  L.  RANSOME. 


154  REINFORCED   CONCRETE  BUILDINGS 

easily  taken  care  of;  however,  it  is  a  common  error  to  have  the 
re-entrant  angles  square  instead  of  chamfered,  and  this  is  of 
particular  importance  where  the  slabs  rest  on  the  beams  or 
girders.  The  removal  of  the  forms  is  greatly  facilitated  by 
having  chamfered  or  beveled  corners,  and  the  finished  struc- 
ture is  less  likely  to  crack. 

A  special  problem  arises  in  connection  with  the  exterior 
construction.  In  modern  practice,  the  columns  and  floors  are 
usually  erected  first,  and  separate  curtain  walls  are  next  placed 
between  the  columns.  These  curtain  walls  may  very  well  be 
utilized  as  deep  beams  at  the  same  time  by  uniting  the  lintel 
below  to  the  curtain  wall  in  a  substantial  manner.1 

The  expansion  and  contraction  of  these  walls  is  readily 
taken  care  of  by  setting  their  ends  into  recesses  in  the  columns 
and  the  windows  may  be  set  into  similar  recesses  above  the  cur- 
tain walls. 

The  cornice  is  tied  to  the  roof  structure  by  means  of  iron 
stubs  projecting  from  the  concrete. 

A  good  design  should  show  not  only  the  location  of  all  pro- 
jecting stubs,  ledges,  recesses,  etc.,  but  also  all  the  minor  open- 
ings for  heat  and  sewer  pipes,  and  the  location  of  pipes  for  gas 
and  electricity.  It  is  a  surprising  fact  that  those  details  are 
so  much  neglected;  it  is  certainly  much  cheaper  and  better  in 
every  way  to  set  proper  sleeves,  etc.,  for  all  such  openings.  Some- 
times, the  pipe-risers  come  up  through  the  columns,  but  this 
practice  is  hardly  to  be  recommended,  as  it  is  both  unsanitary 
and  makes  repairs  and  alterations  difficult  or  impossible.  Spe- 
cial pipe  shafts  may  be  arranged  for;  or  in  some  cases,  the 
exterior  columns  are  made  large  enough  to  accommodate  the 
piping  in  cored  flues  of  ample  size.  Heat  flues  or  ventilation 
may  be  arranged  for  by  having  the  exterior  columns  hollow 
with  register  openings  leading  to  the  several  stories. 

While  the  floors  are  usually  covered  with  cement  finish, 
wooden  floors  are  in  favor  in  many  places.  The  floor  is  nailed 
to  sleepers  laid  on  top  of  the  rough  concrete  base,  and  cinder 
concrete  or  stone  concrete  poor  in  cement  is  run  between  the 
sleepers.  In  many  cases  the  sleepers  apparently  give  good 
satisfaction,  in  others  they  are  soon  destroyed  by  dry  rot.  The 

1  This  construction  forms  the  subject-matter  of  my  U.  S.  Patent,  No. 
694,580,  March  4,  1902.  —  ERNEST  L.  RANSOME. 


FLOOR  SYSTEMS  155 

keeping  qualities  cf  wood  embedded  in  concrete  are  not  well 
known,  but  where  the  concrete  is  in  direct  contact  with  the 
wood,  it  must  certainly  act  as  a  preservative  if  it  has  any  action 
at  all. 

In  hotels  and  similar  establishments,  linoleum  or  carpeting 
over  a  fairly  smooth  cement  base  should  be  very  satisfactory. 

As  a  general  principle  we  must  maintain  that  pipes  should 
not  be  put  inside  the  structural  concrete,  as  they  are  practically 
inaccessible;  the  electric  conduits  may  perhaps  form  an  excep- 
tion to  this  rule.  After  the  steel  has  been  placed  the  electri- 
cian places  his  outlet  boxes  and  connects  them  up,  so  that  the 
conduits  rest  immediately  upon  the  steel.  The  slab  must  then 
be  so  thick  that  the  entire  pipe  is  buried  below  the  neutral  axis 
of  the  slab,  as  otherwise  the  strength  of  the  slab  is  jeopardized. 
Note,  however,  that  when  the  lights  are  suspended  from  the 
bottom  of  a  beam,  the  outlet  box  and  perhaps  a  short  riser 
must  be  placed  before  the  main  tension  steel  is  put  in.  Sewer 
or  gas  pipes  should  always  be  left  exposed;  sleeves  are  placed 
where  they  go  through  the  floor,  so  that  no  cutting  is  required. 
Attention  to  all  such  detail  goes  a  long  way  toward  success  in 
reinforced  concrete  work;  if  the  plumber  is  turned  loose  in  a 
building  to  cut  whatever  holes  he  may  see  fit  he  is  almost  cer- 
tain to  go  through  one  of  the  main  girders,  steel  and  all.  In 
fact,  such  a  case  came  under  the  author's  observation  once.1 

Where  a  wood-floor  finish  is  placed  over  the  concrete,  many 
of  the  pipes  may  of  course  be  concealed  in  the  space  occupied 
by  the  sleepers.  Only  risers  and  outlet  boxes  are  then  placed 
before  the  concrete  is  run.  The  specifications  should  state  in 
detail  who  will  furnish  and  set  the  various  sleeves  required, 
as  there  will  otherwise  be  considerable  friction  between  the 
several  contractors. 

A  number  of  devices  are  on  the  market  by  means  of  which 
shafting  may  be  attached  at  any  place  in  the  building.  All 
such  devices  must  be  decided  upon  in  advance  and  placed  be- 
fore the  concrete  is  run.  If  a  plain  factory  ceiling  is  all  that  is 
wanted,  it  is  convenient  to  place  suitable  bolts  at  intervals, 
with  their  threaded  ends  projecting  from  the  concrete;  timbers 

1  In  the  Academy  of  Science  Bldg.,  San  Francisco,  I  once  caught  a  plumber 
in  the  act  of  cutting  off  the  brick  corbeling  on  which  the  floor  rested.  Many 
such  cases  have  come  to  my  attention  from  time  to  time.  —  E.  L.  RANSOME. 


156  REINFORCED  CONCRETE  BUILDINGS 

are  then  bolted  to  the  ceiling  wherever  wanted,  and  the  shaft- 
hangers  attached  to  the  timbers.  All  threads  must  be  pro- 
tected against  concrete  and  rust.  Sometimes  the  operation  is 
reversed  and  tapped  sleeves  provided  in  the  concrete,  into  which 
the  necessary  bolts  are  screwed.  The  head  of  the  bolt  project- 
ing into  the  concrete  should  be  enlarged  so  that  the  bolt  will 
not  tear  out;  the  pressure  may  be  distributed  over  a  larger 
area  by  means  of  bars  or  plates  underneath  the  head  of  the 
bolt. 

Monolithic  Construction.  The  monolithic  building  is  usu- 
ally erected  a  story  at  a  time.  First:  the  forms  are  set  up, 
forming  a  complete  wooden  shell  for  the  concrete  to  be  depos- 
ited; next,  the  steel  is  put  in  place,  and  the  concrete  run  around 
the  steel  and  within  the  forms.  Simple  as  this  series  of  opera- 
tions may  seem,  there  are,  nevertheless,  a  great  many  details 
to  be  attended  to.  This  is  particularly  true  with  reference  to 
the  form  work,  which  in  itself  absorbs  a  large  proportion  of  the 
total  cost  of  the  building. 

Forms.  The  simplest,  but  in  the  long  run  the  most  expen- 
sive method,  is  to  cut  the  boards  as  needed  and  put  them  to- 
gether box-fashion,  nailing  all  the  joints  securely.  Such  forms 
cannot  be  removed  without  breaking  the  lumber  to  pieces  and 
destroying  a  great  deal  of  the  concrete,  particularly  the  corners, 
and  at  the  present  time  no  experienced  worker  in  reinforced 
concrete  would  consider  using  such  rough  methods. 

The  first  improvement  consisted  in  making  the  slab-panel 
forms  each  in  one  piece,  resting  upon  the  form-panels  for  the 
beam-sides.  All  these  panels  had  cleats  nailed  to  the  side 
facing  away  from  the  concrete,  so  that  each  panel  remained  a 
unit  in  itself  throughout  the  erection  of  the  building,  and  each 
panel  could  then  be  used  over  and  over  again.  For  long  flat 
slabs,  the  panels  rested  upon  joists,  and  sometimes  it  would 
even  be  necessary  to  shore  the  joists  midway  between  the 
beams.  For  the  shorter  spans,  up  to  five  or  six  feet,  the  cleats 
used  under  the  panels  to  hold  them  together  would  usually  be 
sufficient.  Figure  131  shows  schematically  the  most  essential 
parts  of  this  arrangement,  of  which  there  is  a  very  large  num- 
ber of  variations.  Usually,  however,  the  parts  are  so  arranged 
that  the  beam-bottom  with  the  shores  under  same  can  be  left 
in  place  while  the  panels  are  being  removed,  for  the  purpose 


FLOOR  SYSTEMS 


157 


of  keeping  the  beams  supported  for  a  longer  period  than  the 
slab. 

Even  when  nicely  adjusted,  a  falsework  of  this  kind  is 
soon  destroyed  by  the  continuous  prying  and  pounding  required 
to  get  it  loose  from  the  concrete,  and  the  jarring  and  knocking 


FIGURE  131. 

about  while  shifting  from  floor  to  floor.  An  improved  method 
of  centering  was  therefore  devised,  whereby  some  of  these 
objections  would  be  overcome.  This  centering  is  shown  in 
Figure  132,  where  the  long  box  is  split  centrally  down  the  middle, 
each  half  being  held  together  by  the  triangular  cleats,  while 
the  two  halves  are  hinged  together.  The  beam-bottoms  rest 


FIGURE  132. 

upon  cleats  along  the  lower  edge  of  the  box,  and  these  cleats 
also  strengthen  the  bottom  of  the  boxes  where  they  rest  upon 
the  supporting  horses.  At  each  end,  the  boxes  are  closed  by 
means  of  removable  heads.  When  the  forms  are  to  be  removed, 
the  start  is  made  with  the  horses;  next  the  removable  heads 
are  taken  off,  and  finally,  the  boxes  are  collapsed  and  removed. 
Of  course,  the  sketch  shows  the  essential  outlines  only;  such 
portions  as  the  stays  for  holding  the  boxes  expanded,  etc.,  have 
been  omitted. 


158  REINFORCED  CONCRETE  BUILDINGS 

The  main  advantage  of  this  arrangement  rests  in  the  fact 
that  the  benches  or  horses  used  for  supporting  the  boxes  form 
at  once  a  safe  and  even  foundation  for  the  form  work,  and  that 
the  forms  themselves  are  taken  down,  again  put  together,  and 
erected  by  ordinary  labor,  there  being  no  cutting  or  adjustment 
of  any  kind.  Their  use  presupposes  a  standardized  layout, 
and  this,  by  the  way,  is  a  point  to  which  altogether  too  scant 
attention  has  been  given  in  the  past.  It  is  believed  that,  if  a 
number  of  typical  or  standardized  buildings  are  to  be  erected, 
the  simple  attention  to  duplication  of  parts  may  reduce  the 
cost  from  10  to  15  per  cent.,  even  if  the  buildings  are  at 
widely  distant  points.  On  the  rougher  and  simpler  -forms 
of  falsework  it  is  frequently  estimated  that  60  per  cent, 
of  all  lumber  purchased  is  used  on  the  job  for  which  it  was 
bought. 

The  great  secret  of  success  lies  in  attention  to  one  funda- 
mental point:  that  all  parts  must  come  easily  apart  when  the 
forms  are  stripped  from  the  concrete.  Hence  all  shores  must 
rest  on  wedges,  and  all  joists,  etc.,  must  be  keyed  in  place  with 
wedges;  wherever  possible,  bolts  must  engage  in  slotted  holes 
from  which  they  can  be  removed  by  simply  loosening  the  nut 
without  taking  it  off.  Thus,  for  many  purposes,  the  arrange- 
ment shown  in  Figure  133a  is  greatly  superior  to  that  shown  in 


FIGURE  133a.  FIGURE  1336. 


FIGURE  134. 

Figure  133&,  because  a  slight  motion  sidewise  releases  the  bolt 
in  A,  while  the  bolt  in  B  must  be  drawn  through  the  hole. 

The  posts  or  shores  should  have  one  side  of  the  bottom  cut 
away  at  an  angle,  as  shown  in  Figure  134,  to  facilitate  removal. 

Similar  lines  of  argument  lead  to  the  result  that  all  forms 
should  be  made  with  sufficient  "  slip  "  to  leave  the  concrete 
readily,  and  that  the  re-entrant  corners  should  be  beveled; 
in  short,  what  is  good  practice  in  making  patterns  for  cast  iron 
is  also  good  practice  in  making  molds  for  reinforced  concrete. 


FLOOR  SYSTEMS  159 

Amongst  the  more  common  errors  in  the  construction  of 
forms,  attention  is  called  to  the  following: 

Insufficient  stiffness,  so  that  forms  sag  or  bulge. 

Untight  forms,  so  that  cement  is  lost  by  leakage. 

Irregular  thickness  or  width  of  boards,  so  that  bad-looking 
board-marks  result. 

Too  tight  fitting,  necessitating  crowbars  and  sledge-hammers 
when  forms  are  removed. 

Much  time  must  be  devoted  in  the  office  to  the  preparation 
of  details  of  the  forms  and  making  up  of  lumber  schedules;  in 
fact,  it  would  pay  in  many  cases  to  have  the  forms  made  in  a 
well-equipped  carpenter  shop  and  haul  the  forms  to  the  job. 
Much  time  should  also  be  devoted  on  the  job  to  the  inspec- 
tion of  the  forms,  both  during  erection,  concreting,  and  removal, 
to  insure  against  costly  errors.  But  most  of  all,  cleanliness  must 
be  enforced  at  all  cost,  so  that  no  shavings  or  ends  of  boards 
find  their  way  into  the  concrete. 

In  order  to  preserve  the  forms,  the  woodwork  is  frequently 
covered  with  crude  oil,  soap,  or  similar  materials,  and  the  re- 
sults undoubtedly  justify  the  expense.  However,  if  the  ceil- 
ings are  to  be  plastered,  no  oil  must  be  put  on,  as  it  prevents 
the  adhesion  of  the  plaster.  In  that  case,  the  forms  are  simply 
given  a  good  soaking  with  soapy  water  some  little  time  before 
the  concrete  is  run,  and  it  must  be  admitted  that  the  forms 
usually  come  away  from  the  concrete  as  readily  as  when  they 
are  greased. 

While  the  entire  reinforced  concrete  floor  in  many  cases 
may  be  stripped  of  all  form  work  in  a  week's  time  after  the  con- 
crete is  poured,  it  is  not  always  good  practice  to  do  so.  In  the 
summer,  slab-panels  and  beam-sides  may  be  removed  in  about 
one  week,  but  the  beams  and  girders  should  be  shored  up  for 
at  least  three  weeks.  In  the  winter,  the  time  must  be  extended 
considerably.  Altogether,  the  removal  of  the  forms  calls  for 
careful  work  when  it  is  being  done,  and  for  discrimination  as 
to  the  proper  time.  The  strength  of  concrete  depends  greatly 
upon  the  nature  of  the  materials  entering  into  its  makeup; 
hence  what  is  safe  practice  in  one  place  may  be  dangerous  in 
another. 

Reinforcement.  We  have  considered  the  amount  and  re- 
quirements of  the  steel  above;  we  shall  here  consider  briefly  the 


160  REINFORCED  CONCRETE  BUILDINGS 

placing  of  the  steel  on  the  forms,  and  the  preparatory  work  done 
on  it. 

Several  devices  of  merit  are  on  the  market  which  facilitate 
the  bending  and  shaping  of  the  steel;  the  bending  should  be 
done  cold  and  with  so  large  radii  in  the  curves  that  no  injury 
results.  While  the  difficulties  incidental  to  bending  heavy  steel 
bars  to  sharp  corners  usually  prevent  such  practice,  it  is  differ- 
ent with  the  U-bars  and  other  light  steel,  and  many  half  broken 
bars  of  the  lighter  sections  have  without  doubt  found  their  way 
into  important  work. 

Quite  frequently  the  steel  bars  are  assembled  to  suitable 
units,  and  from  every  point  of  view  this  practice  must  be  rec- 
ommended. It  has,  however,  been  argued  that  the  assembling 
of  the  bars  prevents  each  bar  from  sagging  to  its  natural  level, 
so  that  some  bars  are  bound  to  be  stressed  higher  and  earlier 
than  others.  There  is  of  course  some  truth  in  this,  and  per- 
haps some  otherwise  unaccountable  cracks  may  be  explained 
in  this  manner.  The  remedy  is  obvious  —  perfectly  straight 
bars  should  be  used  only,  but  this  follows  from  numerous  other 
reasons  as  well. 

The  steel  may  also  be  bought  ready-made,  assembled  in 
units.  Owing  to  the  cheapness  of  factory  labor  as  compared 
with  field  labor,  and  to  the  better  facilities  found  in  a  well- 
equipped  factory,  ready-made  steel  ought  in  many  cases  to 
be  used  with  a  considerable  saving  in  money  and  time.  How- 
ever, the  steel  yard  affords  an  outlet  for  the  surplus  labor,  and 
for  this  reason  it  is  often  desirable  to  do  the  bending,  etc.,  on 
the  job. 

In  the  early  days  of  reinforced  concrete,  the  beam  steel 
was  placed  after  a  small  amount  of  concrete  had  been  run  in 
the  bottoms  of  the  beams;  similarly  for  the  floor,  the  steel  was 
placed  during  concreting.  At  the  present  time,  the  prevailing 
and  better  practice  is  to  place  all  the  steel  in  the  beams  and  on 
the  floor,  and  not  to  concrete  before  the  steel  has  been  inspected. 
Many  errors  and  much  poor  workmanship  are  thus  eliminated 
(Figure  135). 

Means  should  be  used  for  keeping  the  steel  bars  the  proper 
distance  away  from  the  face  of  the  form.  Metal  clips  or 
cement  blocks  may  be  used,  and  are  much  to  be  preferred. 
The  ordinary  way  is  to  have  a  laborer  raise  the  rods  from  the 


FLOOR  SYSTEMS  161 

forms  with  his  shovel-blade  or  a  hook  made  for  the  purpose, 
but  such  methods  rarely  result  in  satisfactory  work.  There  are 
many  and  strong  reasons  for  keeping  the  slab  bars  one  inch 
from  the  face  of  the  panel  forms,  and  all  other  steel  1J"  to  2" 
from  the  face  of  the  concrete.  The  specified  dimensions  should 
be  adhered  to. 


FIGURE  135.     PLACING  STEEL. 

Morley  Chemical  Laboratory,  Western  Reserve  University,  Cleveland,  O. 
C.  F.  Schweinfurtli,  Architect;  Alexis  Saurbrey,  Engineer. 

Unit  Construction.  If  we  consider  the  list  of  patents  given 
in  a  preceding  chapter,  we  see  that  from  the  earliest  days  of 
the  art,  the  method  of  casting  the  pieces  in  a  yard  and  setting 
them  when  hard  has  been  engaging  the  attention  of  inventors. 
Nor  is  this  strange  when  we  remember  that  our  present  "  rein- 
forced concrete  construction  "  is  a  direct  off-shoot  of  the  arti- 
ficial stone  industry,  and  was  originally  introduced  by  men 
engaged  in  that  kind  of  work  not  less  than  by  men  occupied 
in  the  manufacture  of  monolithic  walls. 

In  the  United  States,  the  actual  use  of  "  Units  "  was  not 
in  much  use  before  1904,  and  the  Textile  Machine  Works, 
erected  in  the  winter  of  1904-5  at  Reading,  Pa.,  was  probably 
one  of  the  first  serious  attempts.  The  Visintini  System  was 
used  for  the  floors  and  girders,  but  the  columns  were  appar- 
ently molded  in  place  as  in  monolithic  work;  this  building  is 


162  REINFORCED  CONCRETE  BUILDINGS 

50'  X  200',  four  stories  high,  and  it  is  stated  that  the  2900  units 
were  put  in  place  for  a  total  cost  of  $586.35  (labor  only),  which 
is  only  about  20  cents  each.  The  completed  building  cost 
7.7  cents  per  cubic  foot. 

In  1906  a  one-story  building  was  erected  for  the  Edison 
Portland  Cement  Co.  at  New  Village,  N.  J.,  the  flat  roof  slabs 
were  cast  on  the  ground,  on  top  of  one  another,  separated  by 
paper.  After  trying  bare,  oiled,  waxed,  and  soaped  paper, 
soaping  just  before  casting  was  found  best  and  most  economical. 
The  roof  girders,  each  50  feet  long,  were  also  cast  in  the  yard. 

The  one-story  building  erected  for  the  Central  Pennsylvania 
Traction  Co.  at  Harrisburg,  Pa.,  in  1909,  had  roof  girders  about 
37  feet  long;  these  as  well  as  the  slabs  were  cast  in  the  yard  and 
set  when  hard.  Another  building  of  exactly  the  same  dimen- 
sions and  similar  design  had  been  erected  close  by  several  years 
before,  by  the  monolithic  method;  it  is  stated  that  the  saving 
in  favor  of  the  unit  type  was  15  per  cent,  in  this  case. 

Recently  the  Unit  Construction  Co.  of  St.  Louis  has  erected 
a  number  of  buildings  up  to  five  stories  high  under  the  Unit 
System;  the  general  design  will  be  apparent  from  Figure  136. 

In  all  the  buildings  just  described,  each  member  has  been 
designed  as  an  individual  carrying  element  without  assistance 
from  the  superimposed  slab.  This  necessitates  the  use  of  T- 
beam  sections  in  order  to  get  the  required  compressive  strength, 
or  the  use  of  extra  deep  beams  or  girders.  In  the  system  to 
be  described  below,  the  slabs  are  utilized  in  compression,  and 
also  used  as  an  extra  means  of  tying  the  entire  building  together, 
while  in  the  cases  just  described,  the  pieces  are  tied  together 
by  virtue  of  bars  projecting  into  pockets  or  open  spaces  in  which 
concrete  is  poured. 

In  the  Ransome  Unit  System,  the  beams,  girders,  and  col- 
umns are  made  in  the  yard,  but  the  -slab  is  cast  in  situ.  The 
first  building  so  erected  was  the  three-story  office  building  of 
the  Foster  Armstrong  Plant  at  East  Rochester,  N.  Y.,  1904-5, 
and  the  same  method  was  subsequently  used  extensively  in  the 
United  Shoe  Machinery  Co.'s  plant  at  Beverly,  Mass.,  for  a 
group  of  four-story  buildings,  60'  X  300'  in  plan,  and  elsewhere. 
(See  American  Machinist,  Sept.  7,  1911;  E.  L.  Ransome:  An 
Innovation  in  Concrete  Building,  from  which  the  following  is 
taken  in  part.) 


FLOOR  SYSTEMS 


163 


Part    "Elevation.^ 


FIGURE  136. 

Unit  constructed  building  with  separate  slab-section  reinforced  with  marginal 
and  central  beams.      (From  the  Engineering  News.) 


164  REINFORCED  CONCRETE  BUILDINGS 

Generally  speaking,  the  beams  and  girders  are  cast  of  a  depth 
equal  to  the  distance  from  the  bottom  to  the  neutral  axis  only, 
and  are  provided  with  projecting  iron  ties.  The  slab  forms 
are  erected  between  the  beams,  which  are  usually  spaced  about 
4  feet  on  centers,  and  rest  upon  3X6  inch  stringers  bolted  to 
the  sides  of  the  beams  (Figure  137).  The  beveled  corners  of 


FIGURE  137. 

the  slab  mold  bring  the  concrete  down  to  the  tops  of  the  beams 
or  girders.  In  the  design,  the  U-bars  must  be  made  with  a 
view  toward  creating  the  required  tie  between  slab  and  beam. 
This  is  easily  and  economically  taken  care  of.  In  addition, 
the  beams  and  girders  must  be  calculated  to  permit  a  working 
load  equal  to  the  weight  of  the  forms,  the  wet  slabs,  the  impact 
from  concreting  apparatus,  and  the  like.  When  this  is  properly 
attended  to  there  is  no  necessity  for  shoring  of  any  kind;  in 
fact,  none  is  used.  It  is  evident  that  the  slab  panels  may  be 
removed  in  a  much  shorter  time  when  so  constructed  than  would 
be  allowable  with  the  monolithic  construction,  because  the 
old  and  properly  seasoned  beams  take  care  of  the  entire  load, 
up  to  the  time  when  the  full  "  live  "  load  is  brought  on  the 
floors. 

The  beams  are  mutually  connected  by  means  of  tie  bars 
placed  in  grooves  in  the  tops  of  the  several  beams,  and  have 
vertical  holes  so  that  the  hooked  ends  of  the  bars  may  engage 
in  holes  in  the  body  of  the  beam.  In  the  more  recent  develop- 
ments of  the  system,  the  reinforcing  rods  project  above  the  tops 
of  the  beams,  and  the  union  between  the  several  pieces  is  effected 
simply  by  a  loose  rod  inserted  alongside  the  tops  of  the  rein- 
forcing rods  and  concreted  in  with  them  when  the  slab  is  run. 
In  neither  case  is  the  beam  considered  as  part  of  a  continuous 
system,  although  with  proper  design  the  continuity  might 
probably  be  taken  advantage  of  to  some  extent. 

The  column  rods  are  made  discontinuous  and  the  tops  and 
bottoms  of  the  column  are  enlarged  so  that  the  concrete  alone 
will  be  sufficient  to  carry  the  weights  at  these  points.  That 
this  method  of  construction  is  adequate,  both  for  columns  and 


FLOOR  SYSTEMS  165 

beams,  is  amply  demonstrated  by  the  absence  of  vibration  under 
heavy  loads  and  high-speed  machinery. 

The  column  details  are  of  considerable  interest.  Aside 
from  the  usual  reinforcement  near  the  sides  of  the  columns,  a 
longitudinal  rod  is  inserted  in  a  central  cored  hole  extending 
lengthwise  through  the  column.  This  hole  runs  from  footing 
to  roof  slab.  These  rods,  therefore,  tie  the  columns  of  each 
story  to  those  of  the  stories  below  and  above.  In  order  to 
unite  the  members,  a  thick,  cream-like  grout  of  1:1  cement 
and  sand  is  poured  down  this  central  hole,  which  is  enlarged 
and  flares  out  at  the  bottom,  so  that  a  secure  and  even  bed 
is  insured  when  the  grout  flows  into  this  larger  space. 

One  of  the  most  important  features  is  that  of  setting  the 
majority  of  the  pieces  dry  and  grouting  the  joints  afterward. 
That  all  joints  really  are  filled  is  readily  ascertained  by  the 
inspector  who  is  instructed  to  see  that  a  small  surplus  of  mortar 
is  forced  out  at  the  bottom  of  the  joints.  For  this  purpose 
small  holes  are  left  in  the  mortar  with  which  the  joints  are 
calked.  This  mortar  is  a  fairly  dry  and  stiff  mixture  applied 
in  the  ordinary  way  with  a  trowel. 

The  buildings  at  Beverly  are  of  rather  complicated  exterior 
design  and  a  number  of  details,  therefore,  have  been  introduced, 
which  would  not  be  found  in  ordinary  plain  factory  work.  Thus, 
the  very  large  flue  columns  in  one  of  the  courts  are  cast  in  place, 
as  the  weight  of  each  exceeds  the  capacity  of  the  derricks. 
Pockets  and  recesses  are  left  in  which  the  beams  and  girders 
are  set.  Otherwise,  all  the  members  are  made  in  advance  and 
set  in  place,  with  the  exception  of  certain  of  the  curtain  walls 
which  are  cast  in  their  final  position  and  keyed  to  the  columns 
with  the  ordinary  recesses. 

Figure  138  shows  the  principal  details  of  a  unit-constructed 
building  erected  under  this  system.  Figure  139  shows  some  of 
the  beams  in  the  process  of  being  set  by  the  derrick.  Figure 
140  shows  one  of  the  stairs  being  set  in  place.  One  flight  was 
built  in  place,  another  set  of  stairs  was  erected  by  the  unit 
method  with  a  saving  of  about  50  per  cent.;  the  more  compli- 
cated the  required  forms  are,  the  greater  is  the  saving. 

The  contractor's  plant  used  at  Beverly  comprises  an  auto- 
matic mixing  plant  whence  the  concrete  is  discharged  into  an 
overhead  hopper  straddling  an  industrial  track.  This  track 


166 


REINFORCED  CONCRETE  BUILDINGS 


parallels  the  building  under  erection  and  serves  the  purpose 
of  bringing  the  concrete  buckets  of  about  one  yard  capacity 
to  either  the  stiff-leg  derrick  used  on  the  building  proper,  or  to 


the  locomotive  crane  used  in  the  casting  yard.  The  columns 
are  cast  in  gangs  of  four  and  other  pieces  in  corresponding  num- 
bers as  required.  The  side  forms  are  removed  in  one  to  two 
days  when  the  weather  is  warm,  and  the  pieces  are  left  undis- 


FLOOR  SYSTEMS 


167 


FIGURE  139.     SETTING  THE  BEAMS. 

United  Shoe  Machinery  Co.,   Beverly,    Mass.     Ernest  L.   Ransome, 
Managing  Engineer. 


FIGURE  140.     SETTING  A  FLIGHT  OF  STAIRS. 
United  Shoe  Machinery  Co.,  Beverly,  Mass.     Ernest  L.  Ransome, 
Managing  Engineer. 


168 


REINFORCED  CONCRETE  BUILDINGS 


turbed  on  their  molding  bed  for  about  ten  days.     The  periods 
are  somewhat  longer  if  the  weather  is  cold. 

If  the  pieces  come  without  the  reach  of  the  stiff-leg  derrick 
they  are  picked  up  and  brought  to  the  building  by  the  loco- 
motive crane.  The  latter  serves  a  multitude  of  purposes  in 


Elevation 


FIGURE  141.     ARRANGEMENT  OF  PLANT,  RANSOMS  UNIT 
SYSTEM.      (FROM  CEMENT  AGE.) 


stripping  and  moving  the  forms,  concreting  in  the  yard,  and  the 
like.  One  of  the  most  interesting  operations  is  the  removal 
of  the  column  cores.  These  are  slightly  tapering  to  facilitate 
drawing;  they  are  six  inches  in  diameter  at  the  top,  four  inches 
at  the  bottom;  they  are  made  of  wood  and  covered  with  sheet 
iron.  In  removing,  the  crane  simply  gives  a  slight  pull  on  the 
core,  which  comes  out  easily  if  the  concrete  is  fairly  green.  No 


FLOOR  SYSTEMS 


169 


instances  have  been  recorded  where  a  column  was  in  the  least 
injured  by  this  treatment. 

The  concrete  is  usually  handled  in  one-yard  bottom-dump 
buckets.  The  molding  of  the  units  presents  very  few  difficul- 
ties, and  the  workmanship  is  greatly  superior  to  that  obtained 
with  the  old  method.  This  high  standard  is  evidenced  in  all 
the  work;  the  lines  are  straighter  and  the  work  generally  truer, 
than  can  possibly  be  obtained  commercially  by  the  older  methods. 


FIGURE   142.     SETTING  A  SLAB. 

United  Shoe  Machinery  Co.,  Beverly,  Mass.      Ernest  L.  "Ransome, 
Managing  Engineer. 

Ii  is  practically  impossible  to  produce  a  large  monolithic  rein- 
forced-concrete  building  commercially  without  some  indications 
of  bulging  forms,  or  of  supporting  shores  having  been  carelessly 
wedged  up,  or  corners  fractured  in  prying  the  forms  loose. 
There  are  no  such  troubles  on  work  of  this  kind,  because  there 
are  no  shores  to  give  way,  no  f^rms  to  bulge. 

It  is  quite  common  to  erect  a  space  of  floor  (for  a  height  of 
one  story)  60  feet  wide  by  40  feet  long  in  one  working  day, 
including  the  setting  of  slab  forms  and  pouring  the  concrete. 
This  also  allows  time  for  calking  the  joints  and  pouring  the 
grouting  into  the  cored  holes  and  recesses. 


170  REINFORCED  CONCRETE  BUILDINGS 

In  one  of  the  one-story  buildings  erected  at  Beverly,  the 
exterior  walls  were  made  of  3"  concrete  panels,  reinforced  with 
\*  twisted  bars  vertically  and  horizontally  spaced  about  two 
feet  apart.  These  panels  were  set  when  eight  days  old,  some  of 
them  with  door  or  window  openings,  or  even  with  the  windows 
concreted  in  place  (Figure  142).  The  wall  panels  were  cast 
on  top  of  one  another  in  stacks,  and  the  layers  separated  by 
means  of  a  heavy  coat  of  common  lime-whitewash. 

In  all  unit  work,  a  proper  margin  must  be  allowed  for  in 
the  design;  all  horizontal  pieces  are  made  from  |"  to  J"  short 
and  all  pockets  or  recesses  are  extra  large;  the  openings  are 
filled  with  grout  afterwards.  Two  transits  were  generally  used 
when  setting  and  plumbing  the  columns. 


CHAPTER  XII 
FOUNDATIONS  AND  PILING 

Foundations.  The  reinforced  concrete  footing  in  common 
use  is  simply  of  pyramidic  shape  with  the  top  removed.  The 
discussion  given  in  Part  II  will  suffice  for  the  design  of  ordinary 
footings;  where  two  or  more  adjacent  footings  are  merged,  the 
design  may  be  of  either  the  flat  slab  type  or  it  may  embody  the 
slab  and  beam  principle. 

Quite  commonly,  the  hole  or  trench  is  excavated  to  the 
approximate  size  of  the  footing,  and  the  concrete  dumped  in 
the  hole  without  forms  of  any  kind.  Such  practice  is  not  to 
be  recommended  unless  the  ground  is  very  stiff.  On  the  con- 
trary, as  the  stability  of  the  entire  building  depends  upon  the 
integrity  of  the  foundation,  the  greatest  care  should  be  taken 
both  with  the  forms  for  the  outside,  with  the  banks  of  the  exca- 
vation so  that  dirt*  will  not  fall  into  the  footing,  and  with  the 
proper  placing  of  the  steel.  The  latter  should  be  protected 
with  not  less  than  4"  of  concrete,  preferably  more,  and  the  con- 
crete around  the  rods  should  be  rich  in  cement  and  dense,  so 
as  to  exclude  water. 

If  metal  base  plates  are  used  under  the  columns,  special  care 
must  be  taken  to  prevent  hollow  places  under  the  plates.  This 
is  best  accomplished  by  setting  the  plates  in  a  thin  grout  1 :  2, 
examining  afterwards  each  plate  by  pounding  with  a  hammer. 

Reinforced  concrete  footings  cannot  conveniently  be  put  in 
under  water,  so  in  a  wet  excavation  it  is  advisable  to  use  plain 
concrete  footings  of  ample  dimensions  to  meet  all  emergencies. 

Piling.  A  number  of  patents  exist  covering  the  various 
methods  of  manufacturing  concrete  piles,  and  several  of  these 
are  operated  by  companies  making  a  specialty  of  concrete  piling. 
To  name  a  few  examples: 

The  Chenoweth  pile,  made  by  rolling  a  sheet  of  fresh  concrete, 
with  fire-fabric  reinforcement,  around  a  central  reinforcement 
or  tube. 

171 


172  REINFORCED  CONCRETE  BUILDINGS 

The  Raymond  pile,  cast  in  a  thin  shell  of  steel,  which  lat- 
ter is  driven  by  means  of  a  collapsible  pile-core.  The  shell 
remains  in  the  ground. 

The  Simplex  pile,  cast  in  a  cylindrical  shell  strong  enough 
to  stand  driving  and  withdrawing,  leaving,  however,  the  point 
or  shoe  behind. 

It  seems,  however,  that  two  methods  are  open  to  the  public: 
(1)  To  drive  a  pile,  withdrawing  it,  and  then  fill  the  hole  with 
concrete;  and  (2)  the  use  of  concrete  piles  molded  in  advance 
and  driven  as  wooden  piles.  The  first  of  these  methods  is  open 
to  several  objections,  so  we  shall  here  give  an  account  of  a  pile- 
driving  job  according  to  the  second  method.1 

The  piles  support  a  one-story  building  with  45-foot  roof 
spans  resting  upon  exterior  piers.  Underneath  each  pier  three 
piles  were  used;  in  addition,  the  chimneys  and  other  founda- 
tions rest  on  piles.  A  total  number  of  484  piles  10"  X  10"  and 
13  feet  long  were  required,  driven  through  fill  and  bog  into 
tenacious  blue  clay.  The  penetration  into  the  clay  was  from 
2  to  3  feet.  The  piles  were  cast  in  the  yard  (Figure  143);  the 
ground  was  levelled  and  tamped,  then  covered  with  a  layer  of 
sand,  one  inch  thick,  and  re-tamped.  OnT  this  bed  the  piles 
were  molded,  the  sand  forming  one  side  of  the  mold.  Two 
sides  were  formed  by  surfaced  boards ;  the  45°  pointed  end  was 
made  by  simply  filling  in  with  molding  sand  to  the  required  slope, 
by  two  If"  thick  pieces  for  the  sides,  and  by  trowelling  the  top 
down  to  the  required  angle.  Otherwise,  the  surface  of  the  pile 
was  smoothed  with  the  back  of  a  shovel.  The  forms  were  re- 
moved in  sixteen  hours  and  immediately  set  again;  thirty  piles 
were  cast  in  one  operation  in  a  gang  mold. 

The  concrete  was  mixed  in  the  proportion  1:1:2,  using 
clean  bank  sand  and  crushed  trap  rock,  pea  size.  This  concrete 
had  an  average  compressive  strength  of  97  tons  per  square 
foot  at  seven  days  when  tested  in  the  compression  machine. 
It  was  found,  however,  that  a  1:2:4  mixture  gave  an  average 
strength  of  104  tons  per  square  foot  in  seven  days,  using  2" 
rock.  This  concrete  could  have  been  used  successfully  and 
would  have  saved  $1.03  per  pile,  reducing  the  cost  from  $6.63 
to  $5.60. 

1  From  a  report  submitted  by  Mr.  B.  C.  Gerwick,  who  acted  as  resi- 
dent engineer  on  the  job  referred  to. 


FOUNDATIONS  AND   PILING 


173 


The  reinforcement  consisted  of  4  —  \"  square  twisted  rods, 
and  a  spiral  reinforcement  of  \"  X  \"  hoop  iron,  4"  pitch. 
Experiments  were  made  with  No.  6  and  No.  8  wire  of  same 
pitch,  but  such  piles  did  not  seem  to  stand  the  driving  as  well. 
In  either  case,  a  \"  twisted  steel  bar  was  used  as  an  extra  col- 
lar reinforcement  near  the  head,  and  the  point  also  had  an  extra 
reinforcing  bar  of  same  section. 


FIGURE  143.     MANUFACTURING  REINFORCED  CONCRETE  PILES. 

United  Shoe  Machinery  Co.,  Beverly,  Mass.      Ernest  L.  Ransome, 
Managing  Engineer. 

The  sand  in  the  bottom  of  the  form  was  first  tamped  and 
sprinkled;  two  inches  of  concrete  were  carefully  placed,  the 
reinforcing  cage  put  in,  and  the  balance  of  the  concrete  placed 
at  once.  As  the  pitch  of  the  hoops  was  4",  they  offered  little 
obstruction  to  the  concreting.  When  the  piles  were  lifted  the 
bottoms  proved  to  be  smooth  and  the  sand  did  not  adhere  to 
the  concrete. 

The  fresh  concrete  was  covered  with  old  sacks  and  kept 
damp  for  three  or  four  days.  In  loading,  the  pointed  end  was 
raised  with  a  bar,  a  rope  sling  slipped  underneath,  and  the  pile 
put  on  the  stone  wagon  by  the  locomotive  crane  available  in  con- 
nection with  other  work.  Four  piles,  each  weighing  about  1350 
pounds,  constitute  a  load,  and  the  haul  is  about  one-half  mile. 


174  REINFORCED  CONCRETE  BUILDINGS 

A  drop-hammer  weighing  1600  Ibs.  was  used  in  driving. 
On  top  of  the  pile  a  cushion  of  several  layers  of  old  fire-hose 
rubber  and  felt  was  placed,  and  over  this  again  a  5"  cast-iron 
block.  An  oak  follower  four  feet  long  rests  on  this  block,  and 
takes  the  blow  of  the  hammer.  The  follower  must  be  of  good 
quality  with  both  ends  banded.  The  fall  of  the  hammer  under 
the  last  blow  was  from  10  to  20  feet,  with  a  penetration  under 
the  last  blow  of  about  f ".  On  an  average,  it  required  about 
seventy  blows  to  drive  a  pile  which  would  penetrate  from  three 
to  four  feet  into  the  clay.  One  pile  was  pulled  by  means  of  a 
lever  and  found  to  be  in  perfect  condition.  The  piles  are  driven 
when  eight  days  old. 

The  crew  consisted  of  foreman,  engineer,  four  pile  driver- 
men  and  two  laborers.  This  crew,  including  the  use  of  the  pile 
driver,  was  hired  for  $30.00  per  day  at  eight  hours.  The  piles 
were,  as  stated  above,  usually  in  groups  of  three,  the  distance 
between  two  adjacent  groups  being  16  feet,  so  that  when  three 
piles  were  driven,  the  driver  had  to  be  shifted  16  feet.  The 
average  time  of  the  shift  was  23  minutes.  The  repairs,  etc., 
totaled  15  minutes  per  day,  and  the  average  time  consumed  in 
actual  driving  was  12  minutes.  The  total  average  time  per 
pile  was  about  20  minutes,  or  24  piles  put  in  every  eight 
hours. 

COST  OF  PILING 

484  piles,  each  13'-0"  long  10"xlO"  in  section 

per  pile 

Grading  casting  yard  for  bottoms $25.45  $    .053 

Cost  of  gang  mold  for  30  piles 

900'  B.M.  spruce  @  $21.00 18.90 

Nails,  lOd 20 

Labor,  carpenters  @  47|c  per  hour 26.65       $45.75  .095 

Setting  forms,  per  gang  of  30 7.00  .233 

Stripping  forms,  per  gang  of  30 72  .024 

Cleaning  and  greasing,  per  gang  of  30 1.16  .038 

Placing  concrete,  per  cu.  ft 0235  .20 

Mixing  concrete,  per  cu.  ft 022  .187 

Labor  on  reinforcement .66 

Cement,  stone,  sand,  and  steel,  cost 3.517 

Hauling  £  mile:  Loading  4  with  crane 26 

Hauling  4 37 

Unloading  4 .11       $     .74  .185 


FOUNDATIONS  AND  PILING  175 

(Cost  of  Piling  —  Continued) 
Driving:  Crew  under  contract  at  $30  per  day, 

Average  day's  work,  24  piles'  $1.25 

Coal,  oil,  and  grease  .125 

Cushion  cap,  $31.40  .065 

Total  cost  per  pile,  in  place  $6.632 

(Overhead  charges  not  included.) 

Where  a  large  number  of  piles  can  be  made  in  a  centrally 
located  yard,  it  sometimes  pays  to  cast  the  piles  vertically. 
Round  forms  can  then  be  used  as  well  as  square. 

A  permanent  plant  of  this  kind  exists  at  Cleveland,  Ohio, 
as  perhaps  elsewhere. 


CHAPTER   XIII 
FINISHING    OPERATIONS 

Corners.  Square  corners  are  contrary  to  the  nature  of  con- 
crete. Projecting  corners  are  difficult  to  make  in  the  first 
place,  as  the  concrete  seldom  penetrates  to  the  very  apex  of 
the  angle;  in  the  second  place  they  are  liable  to  injury  both 
when  the  forms  are  removed  and  while  the  concrete  is  green. 
Once  broken,  they  cannot  be  repaired  so  that  the  patch  looks 
like  the  balance  of  the  work.  The  re-entrant  corner  is  easily 
made,  but  objectionable  for  the  reason  that  a  sharp  dent  in  the 
concrete  very  often  forms  the  starting-point  for  a  crack  which 
might  otherwise  have  been  avoided.  This  is  explained  by  the 
same  observations  made  in  regard  to  cast-iron;  in  addition  the 
form  is  often  locked  to  the  concrete  by  a  sharp  corner  so  that 
the  workmen  use  too  much  force  in  removing  the  forms.  Broken 
corners  are  the  great  drawbacks  in  concrete  construction;  they 
may  easily  be  avoided  by  chamfering  the  forms  so  that  all 
sharp  angles  are  excluded. 

Flat  Surfaces.  All  the  defects  in  the  form  work  will  show 
on  a  flat  concrete  surface;  in  addition,  all  defects  in  the  con- 
crete will  show.  It  is  difficult  if  not  impossible  to  make  per- 
fect forms;  it  is  practically  impossible  to  maintain  the  forms 
in  perfect  condition,  because  the  water  in  the  concrete  is  ab- 
sorbed by  the  wood  in  the  forms,  causing  swelling  and  warping. 
The  marks  left  by  the  forms  are  called  "  board  marks";  if  a 
finished  piece  of  work  is  desired  the  board  marks  must  be  either 
concealed  or  erased.  In  the  first  place,  the  concrete  work  is 
faced  with  various  materials,  such  as  brick,  terra  cotta,  plaster, 
etc.;  in  the  second  case,  the  surface  itself  is  improved  by  tool- 
ing, rubbing,  or  brushing. 

Plastering.  Plaster  usually  comes  off  again  sooner  or  later, 
especially  on  outside  work.  It  should  be  used  for  indoor  work 
only,  and  then  only  in  emergencies;  if  plaster  is  insisted  upon, 

176 


FINISHING  OPERATIONS  177 

the  ceilings  should  be  burned  with  acid,  and  the  form  work 
should  be  made  as  rough  as  possible.  In  that  case,  very  fair 
results  may  be  obtained,  but  plastering  always  remains  an  art 
more  than  a  science,  so  that  skilled  labor  is  a  most  essential 
feature.  There  are  various  plasters  on  the  market,  made  espe- 
cially for  concrete  surfaces.  A  great  deal  of  satisfactory  work 
has  been  done  with  such  material,  but  its  general  use  is  still 
too  recent  to  warrant  absolute  confidence. 

Tile-concrete  construction  is  well  adapted  for  plastering. 

Brick  and  Terra  Cotta  Facing.  The  concrete  must  be  true 
to  line  and  level,  as  it  is  otherwise  difficult  to  put  on  the  brick 
facing,  and  impossible  to  put  on  the  terra  cotta  facing.  For 
brick,  galvanized  wire  wall  ties  are  left  projecting  from  the 
concrete,  say  12"  apart  diagonally,  and  bolts  are  placed  in  the 
concrete  to  receive  the  angle  irons  which  carry  the  brick  work 
over  door  and  window  openings.  For  terra  cotta  the  arrange- 
ment of  the  ties  and  supports  varies  greatly  with  the  design. 
Usually  a  hollow  space  is  left  between  the  concrete  and  the 
terra  cotta  facing;  cement  mortar  deposited  in  this  space  ties 
the  facing  to  the  concrete  behind,  as  the  facing  blocks  have 
projecting  ribs  on  the  back,  while  iron  anchors  project  from  the 
concrete,  so  that  the  whole  is  locked  securely  together.  At 
intervals  supporting  ledges  must  be  arranged  to  transmit  the 
weight  of  the  facing  (which  is  considerable)  to  the  structural 
concrete.  It  is  advisable  to  make  all  ties  of  soft  iron  so  that 
they  will  not  break  when  adjusted.  It  is  very  important  that 
suitable  play  be  provided  for,  as  neither  brick  nor  terra  cotta 
can  be  made  to  exact  dimensions,  while  the  concrete  construc- 
tion is  very  apt  to  vary  slightly  from  the  specified  dimensions. 

Improved  Surfaces.  The  removal  of  the  board  marks  is 
possible  under  one  condition  only,  and  that  is,  that  all  joints 
between  boards  must  be  tight.  The  joints  fill  with  the  finer 
parts  of  the  mixture,  especially  with  cement,  so  that  the  small 
ridges  between  the  boards  are  rich  in  cement.  It  follows  that 
the  concrete  immediately  behind  the  ridge  is  leaner  in  cement 
than  other  parts  of  the  surface,  and  it  is  therefore  softer  than 
the  surface  generally,  so  that  any  mechanical  treatment  of 
the  surface  removes  too  much  at  the  ridges,  forming  small 
grooves  looking  almost  as  bad  as  the  original  ridge.  Hence 
the  joints  must  be  tight,  so  that  no  cement  can  ooze  out,  and 


178 


REINFORCED  CONCRETE  BUILDINGS 


fairly  smooth,  so  that  few  and  small  ridges  only  are  formed; 
the  carpenter  work  must  be  good,  the  forms  must  be  made  of 
good  lumber  and  nailed  securely  to  the  cleats  to  prevent  spring. 
The  completed  form  must  be  coated  with  grease,  vaseline, 
crude  oil,  or,  best  of  all,  a  cheap  grade  of  black  japan.  This 
kind  of  work  is  expensive  when  undertaken  on  a  large  scale; 
to  secure  the  forms  which  retain  the  concrete  so  that  there  will 
be  absolutely  no  deflection,  is  not  easy.  When  now  the  forms 
have  been  completed  to  the  satisfaction  of  the  engineer  the 
concrete  is  deposited  at  once,  as  sun  and  wind  will  destroy  the 
best-made  piece  of  form  work.  The  concrete  must  be  placed 
with  the  greatest  care,  as  every  defect  will  show  in  the  finished 
work.  The  concrete  mixture  must  be  so  gauged  that  there 
is  a  surplus  of  mortar;  usually  1:  2:  3J  or  1:2:3  will  be  found 
suitable.  A  mortar  facing  run  in  the  form  with  the  body  of  the 
concrete  may  be  used  on  work  of  very  large  dimensions  only, 
as  it  is  otherwise  practically  impossible  to  deposit  the  mortar. 
But  a  mortar  without  stone  looks  very  dull  when  tooled.  . 

Tooling.  The  surface  is  bush-hammered  either  by  hand, 
or,  preferably,  with  a  pneumatic  tool.  An  ordinary  chisel  may 
be  used,  but  special  tools  are  sold  for  this  purpose.  The  defects 


FIGURE  144a. 

in  the  concrete  are  brought  out  strongly  by  this  method,  and 
repair  work  looks  very  bad,  especially  in  rainy  weather.  But  if 
the  concrete  was  put  in  right  in  the  first  place  the  effect  is  very 
pleasing,  and  for  large  surfaces  tooling  must  be  considered  as  the 
most  satisfactory  and  most  pleasing  finish  (Figures  144a  and  6). 


FINISHING  OPERATIONS 


179 


Rubbing  with  carborundum  blocks  (or  similar  hard  material) 
is  very  expensive.  The  form  work  must  be  absolutely  first- 
class,  and  the  concrete  must  be  very  hard  before  rubbing  is 
attempted,  but  the  results  justify  the  expense.  The  surface 
is  removed  to  a  depth  of  J"  to  f";  the  grain  of  the  concrete  is 


FIGURE  1446.    GIRLS'  DORMITORY,  LELAND  STANFORD  JR.  UNIVERSITY, 

PALO  ALTO,  CAL. 

The  exterior  construction  and  tl;e  floors  are  of  reinforced  concrete. 
Ernest  L.  Ransome,  Engineer. 

thereby  exposed  with  a  smooth  and  glossy  surface.  It  has  beer* 
found  most  satisfactory  to  rub  the  concrete  down  dry,  in  the 
cases  of  ceilings  and  similar  surfaces  where  large  quantities  of 
water  cannot  be  applied;  certain  kinds  of  cement  floors  are 
manufactured  in  the  same  way,  but  by  a  wet  rubbing.  When 
the  concrete  is  to  be  painted,  a  very  good  surface  may  be  had 
by  this  method  by  taking  only  the  board  marks  off,  leaving  a 
practically,  but  not  entirely,  smooth  surface  (Figure  145). 

Brushing.  The  forms  are  removed  as  soon  as  feasible  and 
the  green  surface  brushed  hard  with  wire  brushes  so  that  the 
mortar  between  the  stone  aggregate  is  removed.  Plenty  of 
water  is  used  in  this  process,  and  the  stones  finally  show  in  re- 
lief on  the  dull  gray  mortar  backing.  A  sparkling,  many- 
colored  aggregate  is  used,  and  the  effect  is  very  good,  although 
perhaps  a  little  artificial. 

Unfinished  surfaces  are  sometimes  used  for  factory  build- 
ings, stables,  etc.,  and  all  degrees  of  work,  from  very  good  to 
very  poor,  may  be  found.  Occasionally  an  effort  is  made  to 
improve  such  surface  by  rubbing  cement  mortar  into  the  pores 
at  once  upon  removal  of  the  forms,  and  then  rubbing  the  entire 


180 


REINFORCED  CONCRETE  BUILDINGS 


surface  down  with  cement  bricks,  or  sometimes  hardwood  blocks. 
If  the  purpose  is  to  fill  the  pores  only  there  can  be  no  criticism 
of  this  method,  provided  all  the  surplus  mortar  is  again  removed; 


FIGURE  145.  MONOLITHIC  CONCRETE  STAIRS  AND  RAIL,  CAST  IN 
ONE  PIECE  THREE  STORIES  HIGH.  CONCRETE  WAS  RUBBED  WITH 
CARBORUNDUM  AND  PAINTED. 

Morley  Chemical  Laboratory,  Western  Reserve  University.  C.  F. 
Schweinfurth,  Architect;  Alexis  Saurbrey,  Engineer. 

often  the  mortar  is  allowed  to  remain  on  the  surface  as  a  thin 
film,  in  which  case  more  or  less  pealing  is  bound  to  follow. 
Many  arches  and  abutments  throughout  the  country  have 
been  provided  with  a  coat  of  this  kind,  and  there  is  hardly  any 


FINISHING  OPERATIONS  181 

locality  where  samples  of  this  work  may  not  be  found,  showing 
the  disgraceful  results  obtained. 

Cement  Finish.  Only  in  exceptional  cases  may  the  rough 
concrete  floor  be  used,  partly  because  the  surface  is  too  coarse, 
partly  because  it  wears  out  too  rapidly,  partly  because  it  forms 
part  of  the  structure  itself  and  therefore  needs  protection  against 
wear.  The  rough  floor  is  therefore  covered  with  a  sheet  of 
cement  mortar,  called  "  cement  finish."  It  may  be  applied  to 
the  green  concrete  surface  as  soon  as  it  is  hard  enough  to  allow 
walking  on  it,  or  at  any  time  afterwards.  In  the  first  case  a 
good  bond  is  assured  by  simple  precautions,  such  as  removal 
of  the  "  slam,"  a  white  scum  forming  on  top  of  concrete  laid 
with  an  excess  of  water.  If  the  concrete  is  hard  and  old,  the 
surface  must  be  cleaned  with  muriatic  acid,  water,  and  scrub- 
bing brushes,  as  no  cement  finish  will  stick  to  a  dirty  surface. 
The  finish  is  put  on  in  a  rather  dry  condition,  like  soft  dough; 
about  |"  or  1"  thick  as  a  minimum,  and  up  to  2"  thick  as  a 
maximum.  It  is  trowelled  to  a  hard  surface  in  order  to  ensure 
good  wearing  qualities,  and  it  may  be  divided  into  panels  or 
not,  according  to  circumstances.  The  object  in  dividing  the 
surface  into  panels  is  purely  ornamental;  cracks  may  be  avoided 
by  dividing  the  base  as  well  as  the  surface  into  suitable  blocks. 
A  structural  reinforced  concrete  floor  is  not  readily  divided  in 
this  manner;  in  fact,  one  of  the  objects  of  good  design  is  to 
make  the  floor  continuous  as  far  as  possible.  It  is  therefore 
proper  to  divide  basement  floors,  sidewalks,  and  similar  pieces 
into  blocks  by  deep  and  wide  separations,  while  the  finish  on 
a  reinforced  floor  may  as  well  be  laid  in  one  continuous  sheet. 
Thereby  is  also  avoided  the  breaking  of  the  edges  of  the  individ- 
ual blocks,  so  likely  to  take  place  under  heavy  trucks  in  ware- 
houses. A  surface  laid  in  this  manner  will  show  all  the  cracks 
in  the  base  below,  and  for  this  reason  as  well  as  on  general 
principles,  all  care  should  be  taken  to  avoid  cracking.  These 
are,  as  stated  above:  proper  arrangement  of  principal  and  sec- 
ondary reinforcement,  bevelling  of  all  re-entrant  corners  between 
beams  and  slabs,  protection  against  wind  and  sun,  liberal 
sprinkling,  and  avoidance  of  premature  loading  and  jarring. 
Great  care  should  be  taken  when  the  forms  are  removed;  in 
fact,  a  large  number  of  "  unaccountable  "  cracks  are  due  to 
carelessness  in  removing  the  forms. 


182  REINFORCED  CONCRETE  BUILDINGS 

To  avoid  cracks  entirely  is  hardly  possible,  especially  in 
tile-concrete  floors,  where  top  cracks  are  very  frequent,  due 
to  the  unyielding  nature  of  the  tiles.  Such  floors  are  better 
provided  with  a  wood  floor  on  top  of  the  concrete. 

The  mixture  used  for  the  finish  should  be  one  part  of  cement 
to  two  parts  of  selected  sand,  whereby  is  meant  a  sand  with 
particles  well  graded  from  fine  to  coarse,  clean,  and  sharp. 
The  largest  particles  should  not  exceed  the  1/4"  to  3/8 "  ring. 
The  finer  the  sand,  the  easier  it  works  under  the  trowel,  so  that 
fine  sand  is  the  preference  of  the  cement  finisher,  to  the  injury 
of  the  work.  Aside  from  proper  materials,  skilled  cement  fin- 
ishers are  indispensable  to  good  results.  The  engineers'  super- 
vision of  the  workmanship  is  usually  confined  to  the  results,  as 
few  engineers  are  sufficiently  well  posted  on  cement  finish  to 
supervise  the  details  of  the  workmanship. 


CHAPTER   XIV 
FIREPROOFING  AND   FIRES 

No  building  is  absolutely  "  fireproof,"  and  the  most  that  can 
be  accomplished  is  to  retard  the  spread  of  the  fire  to  such  an 
extent  that  the  fire  can  be  brought  under  control  before  the 
barriers  are  destroyed.  But  this  is  only  one  side  of  the  question, 
for  in  many  cases  more  damage  is  caused  by  smoke  or  water  than, 
by  the  fire  itself.  To  prevent  the  smoke  from  penetrating  to 
portions  of  the  building  not  affected  directly  by  the  fire,  is  usually 
impossible,  but  much  may  be  done  to  prevent  the  water  from 
leaking  down  into  the  stories  below  the  fire.  We  encounter 
here  a  much  neglected  problem:  In  most  cases,  pipes  for  heat, 
sewerage,  etc.,  are  carried  through  the  floors  by  means  of  open 
sleeves,  and  the  water  naturally  finds  its  way  out  through  all 
these  holes  in  the  floors.  However,  if  we  consider  a  perfectly 
waterproof  floor  without  means  of  escape  for  the  water,  the  load 
on  the  flooded  floor  might  easily  exceed  the  capacity  of  the  struc- 
ture to  a  dangerous  degree. 

While  therefore  many  owners  of  reinforced  concrete  buildings 
carry  no  insurance  on  the  building  itself,  it  is  not  advisable  to 
neglect  the  insurance  on  the  contents,  except  where  they  are  of 
such  a  nature  that  they  are  not  easily  injured  by  water,  smoke, 
or  heat.  Much  will  also  depend  upon  the  character  of  windows, 
partitions,  stair-  and  elevator- wells.  In  the  majority  of  cases, 
the  so-called  "  fireproof "  building  is  equipped  with  wood 
trimmings,  plain  glass  in  wooden  casings,  and  has  a  wood  floor 
over  the  concrete  base.  While  in  such  cases  the  reinforced 
concrete  escapes  injury,  the  contents  are  usually  a  total  loss, 
frequently  with  loss  of  human  lives.  There  is,  without  doubt, 
room  for  great  improvement  along  these  lines.  The  arrange- 
ment of  these  matters,  as  well  as  those  pertaining  to  stairwells 
and  elevator  openings  is,  however,  beyond  the  scope  of  this  book. 

Turning  now  to  the  concrete  itself,  it  is  admitted  that  no 

183 


184  REINFORCED  CONCRETE  BUILDINGS 

material  is  absolutely  fireproof,  and  concrete  as  well  as  other 
materials  must  finally  fail  under  a  long  and  severe  fire  test. 
But  each  particle  in  the  crystallized  concrete  contains  chemically 
bound  water  which  is  given  off  under  high  temperature,  and 
the  temperature  of  the  concrete  itself  is  thereby  prevented  from 
reaching  a  high  intensity  throughout  the  mass.  Concrete  itself 
is  a  good  conductor  of  heat  compared  with  the  true  insulating 
materials. 

The  concrete  surrounding  the  steel  must  be  made  so  thick 
that  a  large  part  of  it  may  lose  its  water  (and  thereby  its  strength) 
without  injuring  the  strength  of  the  concrete  touching  the  steel, 
as  otherwise  failure  would  result.  But  large  amounts  of  concrete 
are  expensive:  practice  has  therefore  settled  upon  2"  of  protec- 
tion on  columns  and  girders  or  beams,  and  I"  on  slabs.  These 
thicknesses  are  entirely  arbitrary  and  may  be  varied  according 
to  location  and  exposure,  but  it  will  be  seen  that  they  are  also 
structural  minima,  and  that  with  less  concrete  around  the  steel 
there  can  be  absolutely  no  bond,  and  a  small  enough  factor  of 
safety  as  far  as  the  workmanship  is  concerned.  It  happens 
quite  often  that  these  specified  minima  are  still  further  decreased 
by  carelessness  on  the  part  of  the  concreting  gang,  so  that  rust 
spots  show  through  the  concrete,  or  the  steel  is  even  exposed 
to  view.  Such  conditions  are  always  indications  of  workmanship 
of  the  poorest  class. 

The  parts  most  exposed  to  the  attack  of  fire  are  the  projecting 
corners.  Experience  has  shown  that  rounded  or  chamfered 
corners  are  much  less  liable  to  attack  than  a  plain  square  corner ; 
in  addition,  square  corners  are  almost  always  more  or  less 
fractured  when  the  forms  are  removed.  In  warehouses  or  other 
buildings  where  heavy  stuff  is  handled,  the  lower  part  of  the 
columns  should  have  extra  protection,  such  as  angle  iron  guards 
for  the  corners,  or  even  an  iron  mantel  surrounding  the  concrete 
entirely.  The  same  is  true  of  thresholds  and  stairways;  the 
steps  of  the  latter  are  often  protected  with  some  patented  metal 
covering,  of  which  the  nosing  piece  is  the  most  essential  part. 
The  elevator  hatches  should  also  have  a  proper  protection; 
angle  iron  guards  are  easily  fastened  and  effective. 

In  addition  to  these  obvious  safeguards,  we  have  an  excellent 
method  of  increasing  the  fire  resistance  of  concrete.  It  simply 
consists  in  adding  a  small  amount  of  salt  to  the  water  with  which 


FIREPROOFING  AND   FIRES  185 

the  concrete  is  mixed.  This  fact  was  proved  in  a  peculiar 
manner:  During  the  erection  of  the  Bayonne,  New  Jersey, 
warehouse  for  the  Pacific  Coast  Borax  Co.  in  the  winter  1897- 
1898,  experiments  were  made  with  salt  as  a  frost  preventive, 
the  work  being  carried  on  in  very  severe  weather,  with  tempera- 
tures sometimes  below  zero.  In  1902,  the  building  went  through 
an  exceptionally  hot  fire,  started  from  a  burst  oil  main  in 
the  basement,  which  soon  was  flooded  with  burning  oil.  On 
the  upper  floors,  combustible  materials  of  all  kinds,  including 
heavy  barrels  and  boxes,  added  to  the  fire,  yet  the  concrete  came 
out  of  the  fire  with  hardly  any  damage,  and  the  concrete  work 
of  the  entire  building,  200'  X  240',  and  partly  four  stories  high, 
was  repaired  for  less  than  $1000.  Quantities  of  fused  cast-iron 
from  the  machinery  and  copper  from  the  dynamos  and  motors 
were  in  evidence  after  the  fire  (See  Iron  Age,  May  28,  1902), 
showing  that  the  fire  must  have  been  unusually  hot.  (Figure 
146  shows  a  block  of  fused  cast-iron  from  this  fire). 

The  increased  fire-resistance  due  to  an  admixture  of  salt  has 
also  been  demonstrated  on  test  specimens  made  for  the  purpose. 

The  general  behavior  of  reinforced  concrete  in  conflagrations 
has  been  very  satisfactory  in  the  Pittsburgh  and  Baltimore 
fires,  where  but  few  reinforced  concrete  buildings  were  within 
the  fire-swept  area;  the  most  convincing  proofs  were  however 
furnished  in  the  San  Francisco  earthquake  and  conflagration, 
where  buildings  of  all  kinds  suffered,  but  those  of  reinforced 
concrete  less  than  any  others.  Tests  without  number  have  been 
made  to  determine  the  conductivity  and  fire-resistance  of  con- 
crete. As  a  result,  it  may  be  stated  that  the  better  the  concrete 
is  made  originally,  the  better  it  will  be  adapted  for  fireproofing 
purposes,  and  a  four-inch  concrete  wall  may  be  exposed  to  the 
hottest  fire  for  hours,  on  one  side,  while  the  other  side  remains  so 
cool  that  the  hand  may  be  placed  against  it  without  fear.  The 
use  of  reinforced  concrete  for  heat  flues  and  chimneys  is  justified 
from  this  fact. 

At  the  present  time,  a  "  fireproof  "  floor  may  be  built  in  one 
of  three  ways:  (1)  By  combining  steel  and  concrete,  whether 
the  steel  be  in  the  form  of  a  reinforcement,  or  as  an  independent 
skeleton.  (2)  By  combining  steel  and  tile,  in  which  case  the 
steel  forms  the  well-known  skeleton  so  commonly  used  in  the 
modern  skyscraper.  (3)  By  combining  steel  with  both  tile  and 


186 


REINFORCED  CONCRETE  BUILDINGS 


concrete  in  various  ways.  (There  is  indeed  a  fourth  method, 
by  suspending  brick  arches  between  steel  beams,  but  this  is  in 
little  use  at  present  except  for  special  structures.) 


FIGURE  146.  FUSED  CAST-IRON  PULLEY  FROM  THE 
BAYONNE  FIRE. 

The  competition  is  therefore  between  concrete  and  hollow 
tile,  both  or  either  in  combination  with  steel.  Wherever  put 
to  the  test,  concrete  appears  to  have  carried  the  day.  Two 
reasons  suggest  themselves  for  this  fact:  (1)  The  expansion  of 
concrete  and  of  steel  is  practically  the  same,  while  the  expansion 
of  tile  is  different  from  that  of  steel,  so  that  there  is  a  tendency 
to  readjustment  under  fire  in  the  latter  case,  and  none  in  the 


FIREPROOFING  AND  FIRES  187 

former;  and  (2)  the  hollow  tiles  in  common  use  are  very  poor 
conductors  of  heat,  so 'that,  while  the  steel  is  protected  in  an 
excellent  manner  while  the  tiles  stand  up,  the  unequal  expansion 
of  the  several  parts  of  the  same  tile  causes  the  lower  flange  to 
break  off,  especially  when  suddenly  cooled,  thus  exposing  the 
steel.  The  proofs  of  this  statement  are  ample  and  convincing 
and  may  be  seen  by  reference  to  the  photographs  in  the  Govern- 
mental report  on  the  San  Francisco  fire. 

It  must  be  understood  that  lath  and  plaster  construction  is 
not  included  as  a  fireproof  material,  and  has  no  value  as  such. 

In  closing  this  paragraph,  we  must  call  attention  to  the  ever 
present  danger  attending  the  use  of  reinforced  concrete  buildings 
veneered  with  brick  or  similar  material,  where  the  horizontal 
supports  are  exposed  over  the  window  openings  as  is  nearly 
always  the  case.  A  hot  flame  through  the  window  would 
probably  injure  the  supports  and  wall-ties  sufficiently  to  cause 
parts  of  the  veneer  to  fall,  although  no  such  accidents  have  ever 
come  to  our  attention. 

Other  considerations  also  lead  us  to  doubt  the  continued 
stability  of  thin  veneer  walls,  and  there  seems  to  be  no  good 
reasons  for  their  extensive  use. 


CHAPTER  XV 

REPAIRS  TO  EXISTING  BUILDINGS 

THE  general  wear  and  tear  on  a  well-constructed  reinforced 
concrete  building  is  insignificant  and  confined  to  the  finish  coat 
of  the  floor.  The  repairs  consist  in  careful  removal  of  the 
worn  surface,  thorough  cleaning  of  the  floor,  eventually  with 
weak  muriatic  acid,  —  the  application  of  a  bonding  substance 
such  as  Livingstone,  Ransomite,  or  similar,  and  the  placing  cf 
a  new  surface  coat. 

It  happens,  however,  occasionally  that  carelessness  when  the 
work  was  made  causes  trouble,  and  a  brief  description  of  some 
cases  of  this  kind  may  be  of  interest. 

Cracking  of  the  floor  slab  may  be  due  to  a  number  of  causes: 
concrete  poorly  proportioned  with  accompanying  excessive 
contraction,  too  rapid  drying  out  of  the  concrete,  etc.  All  cracks 
may  be  repaired  that  are  caused  by  a  natural  adjustment  when 
a  stable  condition  has  been  reached,  by  simply  cutting  out  the 
cracks,  dove-tailing  the  bonding  surface  on  both  sides,  and  filling 
in  with  fresh  concrete.  Many  slabs  are  broken  when  the  forms 
are  removed,  although  the  crack  does  not  appear  for  some  time. 
Cracking  of  beams  or  girders  is  usually  due  to  careless  or  pre- 
mature removal  of  the  forms.  A  gaping  crack  is  an  indisputable 
sign  that  the  reinforcement  has  slipped,  and  it  is  then  a  question 
of  removing  the  entire  beam  and  putting  in  a  new  one.  In  that 
case,  pockets  are  left  for  the  new  beam  at  each  end,  and  the 
new  concrete  tied  as  well  as  possible  to  the  old  work. 

If  upon  examination  the  steel  is  found  too  high  in  the  beam, 
as  sometimes  happens,  it  is  possible  to  cut  away  the  bottom  por- 
tion of  the  beam  for  its  entire  length,  and  to  put  in  a  new  bottom 
with  proper  reinforcement,  leaving  the  old  steel  in  place.  The 
new  bottom  is  tied  to  the  old  beam  by  means  of  frequent  U-bars 
which  are  concealed  in  vertical  grooves  cut  for  the  purpose  in 
the  sides  of  the  beam ;  the  upper  ends  of  the  U-bars  are  carefully 
anchored  in  new  portions  of  the  slab  inserted  in  spaces  made  to 

188 


REPAIRS   TO  EXISTING  BUILDINGS  189 

receive  them.  The  new  reinforcement  must  extend  well  over  the 
supports,  and  firm  anchorage  must  be  provided  for  it.  Hollow 
spaces  in  the  columns  are  repaired  by  cutting  the  poor  concrete 
entirely  away,  cleaning  the  surfaces,  and  pouring  new  concrete 
in.  It  must  here  be  observed  that  the  concrete  surfaces  are 
made  to  slope  to  such  an  extent  that  all  air  can  escape,  preferably 
through  a  vent  on  the  opposite  side  of  the  funnel  through  which 
the  new  concrete  is  poured.  The  purpose  of  the  vent  and  funnel 
is  to  make  certain  that  the  concrete  fills  all  cavities  by  putting 
the  fresh  concrete  under  pressure.  The  surplus  stuff  is  dressed 
off  and  the  surfaces  smoothed  down. 

Under  no  circumstances  should  cutting  in  concrete  be  done  except 
in  the  presence  of  a  reliable  engineer  who  understands  the  structural 
importance  of  each  member,  and  proper  shoring  must  be  put  under 
the  beams,  etc.,  before  the  cutting  is  proceeded  with. 

When  cutting  holes  in  structural  concrete  of  any  kind,  the 
lighter  hammers  and  chisels  should  be  used  in  preference  to  the 
heavy  tools,  and  many  light  blows  rather  than  a  few  heavy  ones 
should  be  insisted  upon  for  the  reason  that  heavy  blows  have 
considerable  shattering  effect  on  the  concrete,  especially  in  the 
first  few  weeks  after  pouring.  Hence  the  pneumatic  drill  is  to 
be  preferred  where  obtainable,  even  if  at  much  greater  cost, 
especially  when  putting  in  new  bolt  holes,  etc.,  in  great  number. 
But  drilling  into  the  bottoms  of  beams  and  girders,  or  into  the 
sides  of  hooped  columns,  should  not  be  allowed  when  avoidable, 
and  it  is  quite  often  possible  to  confine  the  drilling  to  the  slabs 
and  the  sides  of  the  beams. 

Sometimes,  an  annoying  and  troublesome  condition  arises 
from  the  fact  that  the  laitance,  or  dead  cement,  accumulates 
in  the  beam-bottoms.  This  can  happen  only  where  the  concrete 
has  been  made  with  a  surplus  of  water,  and  the  water  has  been 
allowed  to  run  ahead  of  the  concrete  into  the  bottoms  of  the 
beams,  carrying  considerable  amounts  of  cement  with  it.  The 
water  and  cement  form  a  soapy,  white  substance  which  never 
sets  up,  and,  after  a  while,  large  cakes  drop  from  the  beam-bot- 
toms, sometimes  an  inch  thick.  The  only  efficient  manner  of 
repairing  is  by  putting  in  a  new  beam-bottom,  tying  the  new  con- 
crete to  the  old,  and  this  procedure  is  very  expensive,  although 
cheaper  in  the  long  run  and  far  better  than  repairs  with  cement 
mortar  troweled  on. 


190  REINFORCED  CONCRETE  BUILDINGS 

In  exceptional  cases,  poor  foundations  cause  unequal  settle- 
ment and  cracks.  If  the  footings  finally  adjust  themselves  to 
a  permanent  level,  the  cracks  may  be  repaired  as  described  above; 
but  even  in  that  case,  the  building  has  lost  considerably  in  carry- 
ing capacity,  especially  if  constructed  with  continuous  beams 
and  girders.  In  one  case,  it  became  necessary  to  install  new 
footings,  columns,  and  girders  alongside  the  old  work,  but  in 
that  case,  the  original  footings  had  not  been  brought  down  to 
the  proper  level,  and  the  girders  had  been  erroneously  designed. 

After-treatment  of  the  surface  of  cement  finish  may  be  desirable 
in  exceptional  cases  to  further  the  hardening,  in  which  case  a 
wash  of  equal  parts  of  water  and  the  commercial  solution  of 
Silicate  of  Soda  is  applied.  Silicate  of  Potash  may  be  substituted 
for  the  Soda,  but  the  best  results  are  obtained  by  using  a  wash 
of  two  parts  of  water  with  one  part  of  Silicate  of  Soda  and  one 
part  of  Silicate  of  Potash ;  the  two  latter  in  the  ordinary  commer- 
cial solution. 

A  final  wash  with  Chloride  of  Calcium  is  very  desirable, 
especially  if  there  is  no  free  lime  in  the  cement. 

For  repairs  to  small  cracks,  a  mixture  of  Silicate  of  Soda  and 
Chloride  of  Calcium  may  be  poured  into  the  cracks,  but  as  this 
solution  sets  very  rapidly,  Alum  may  be  used  instead  of  the 
Chloride  of  Calcium  as  this  mixture  sets  much  slower. 


CHAPTER   XVI 

ACCIDENTS 

A  GOOD  reinforced  concrete  building  is  as  permanent  as  any 
type  of  construction  known  today,  and  where  a  building  of  this 
kind  has  been  taken  in  use,  it  has  never  been  known  to  fail, 
with  one  or  two  exceptions  where  the  design  was  faulty,  or 
where  the  foundations  were  entirely  inadequate.  In  the-  very 
few  cases  where  reinforced  concrete  buildings  have  been  pur- 
posely demolished  the  task  has  proved  an  arduous  one,  as 
for  instance  the  seven-story  building  of  the  Baltimore  News 
which  was  taken  down  in  the  spring  of  1911  to  give  room  for  a 
larger  structure. 

It  would  be  possible  to  enumerate  a  number  of  minor  mishaps, 
serious  enough  to  those  whom  they  affected,  but  similar  to  those 
which  do  occur  in  all  lines  of  building  construction,  whether 
brick,  steel,  or  concrete.  Here  we  will  limit  ourselves  to  the 
few  disasters  which  attracted  universal  attention,  and  give  a 
brief  account  of  the  cause  in  each  case,  in  so  far  as  the  cause  is 
known.  It  will  be  appreciated  that  the  tangled  mass  of  debris, 
and  the  more  or  less  colored  account  of  the  actual  conditions 
given  by  the  parties  directly  affected,  furnish  but  poor  material 
upon  which  to  base  an  unbiased  opinion. 

The  collapse  of  a  portion  of  the  Amsden  Block  at  South  Fram- 
ingham,  Massachusetts,  in  July,  1906,  has  been  traced  to  the 
settling  of  the  foundations,  and  inasmuch  as  the  interior  con- 
struction consisted  of  reinforced  concrete  only  for  the  slabs  and 
fireproofing,  the  beams  being  of  steel,  and  the  columns  of  cast- 
iron,  there  is  no  reason  to  believe  that  the  reinforced  concrete 
was  to  blame  for  the  failure. 

The  Bixby  Hotel,  at  Long  Beach,  California,  was  a  building 
H-shaped  in  plan,  with  reinforced  concrete  construction  of  the 
tile-and-concrete  variety  for  the  interior,  and  the  usual  concrete 
skeleton  for  the  exterior  construction.  A  large  portion  of  the 

191 


192  REINFORCED  CONCRETE  BUILDINGS 

bar  of  the  "  H  "  fell  while  the  roof  was  being  concreted,  on 
November  9,  1906.  Questions  as  to  design  and  unit  stresses 
assigned  to  the  columns  have  been  raised,  and  it  seems  probable 
that  some  of  the  columns  failed  in  one  of  the  upper  stories. 
However  this  may  be,  premature  removal  of  the  falsework 
undoubtedly  entered  into  the  causes  of  the  collapse. 

The  Eastman  Kodak  Company's  building  at  Rochester,  New 
York,  was  partly  demolished  by  a  sudden  failure  on  November 
21,  1906,  while  the  waterproofing  was  being  put  on  the  roof, 
which  at  that  time  was  seventeen  to  eighteen  days  old.  The 
initial  failure  seems  to  have  been  traced  without  doubt  to  column 
No.  47  which  at  that  time  was  about  three  weeks  old,  but  there 
also  seems  to  have  been  more  or  less  neglect  on  the  job  with 
reference  to  the  proper  placing  of  the  column  reinforcement  and 
some  of  the  columns  had  considerable  amounts  of  saw-dust  and 
chips  of  wood  embedded  in  the  concrete.  The  shores  were 
probably  being  removed  in  some  portions  of  the  building  during 
the  time  preceding  the  collapse. 

The  Bridgman  Bros,  building  in  Philadelphia  was  partly 
wrecked  on  July  9,  1907,  when  some  foreign  laborers  removed 
all  the  shores  under  the  roof  which  at  that  time  was  only  5|  days 
old,  owing  to  a  misunderstanding  of  orders.  The  falling  por- 
tions of  the  roof  carried  with  it  all  the  floors  directly  below, 
except  a  portion  of  the  first  floor,  which  partly  withstood  the 
shock  of  the  falling  concrete. 

Failure  under  test  load  took  place  in  the  roof  of  the  reservoir, 
at  the  United  States  Naval  Academy,  Annapolis,  Maryland,  in 
December,  1908.  The  footings  apparently  had  been  put  in  on 
very  wet  clay,  and  these  footings  were  without  reinforcement 
except  for  the  wirecloth  which  was  used  to  reinforce  the  floor, 
and  which  was  run  into  the  footings,  being  depressed  under  the 
columns  to  near  the  bottom  of  the  footings  as  well  as  possible. 

On  April  7,  1910,  a  car  barn  then  nearly  completed,  and 
belonging  to  the  Shore  Line  Electric  Company,  at  Saybrook, 
Connecticut,  partly  collapsed  owing  to  the  premature  removal 
of  the  forms  under  the  roof. 

Finally,  the  Henke  Building  in  Cleveland,  Ohio,  was  entirely 
destroyed  by  collapse  on  November  22,  1910.  (Figure  147). 
The  building  was  four  stories  high,  and,  with  the  exception  of  a 
few  of  the  old  brickwalls  used  for  the  outside,  the  entire  building 


ACCIDENTS 


193 


fell  so  as  to  fill  the  basement  level  with  the  sidewalk  of  the 
street.  The  roof  was  just  completed,  and  it  has  been  suggested 
that  work  was  going  on  in  the  building  on  the  removal  of  the 
few  remaining  shores  in  the  second  story  from  the  top;  there 
were  several  indications  of  column  failures  in  that  story.1 


FIGURE   147.      WRECK  OF  THE  HENKE  BUILDING  IN  CLEVELAND. 
Photo  by  Alexis  Saurbrey,  who  examined  ruins  for  owner. 

In  nearly  every  one  of  these  cases,  serious  errors  in  regard  to 
supervision    and   workmanship    have  been  proved,  but  it  has 

1  While  this  was  in  the  press,  the  current  issues  of  engineering  papers 
reported  the  failure,  on  Dec.  6,  1911,  of  a  three-story  building  under  erection 
for  the  Prest-O-Lite  Company,  at  Indianapolis,  with  considerable  loss  of 
life.  The  building  was  of  the  beam  and  girderless  type,  but  the  details 
are  not  available. 


194  REINFORCED  CONCRETE  BUILDINGS 

not  been  possible,  as  far  as  known,  to  connect  the  neglect  with 
the  actual  causes  of  the  collapse.  Nearly  all  of  these  buildings 
fell  in  the  spring  or  in  the  fall,  when  the  setting  of  the  concrete 
is  greatly  retarded  by  the  cold  weather,  and  even  if  the  days  may 
be  quite  warm,  the  nights  are  cool,  and  the  water  used  for  the 
concrete  very  likely  quite  cold.  It  is  easy  to  say  that  if  the  shores 
had  been  left  in  a  few  weeks  longer,  the  failures  would  not  have 
occurred,  but  it  is  no  easy  matter  to  prove  such  assertions. 

Attention  is  called  to  the  circumstance  that  the  failures  have 
frequently  suggested  weakness  in  certain  columns,  and  in  all 
such  cases,  the  horizontal  column  reinforcement  has  been  found 
grossly  inadequate  or  even  missing.  It  is  a  positive  necessity 
to  provide  proper  ties  or  hoops  in  the  columns,  not  one  or  two 
column  Diameters  apart,  but  two  to  three  inches  apart,  thoroughly 
binding  the  loose  ends  of  the  hoops  together  so  that  they  cannot 
slip.  In  addition,  no  shores  should  be  removed  before  the 
column  sides  have  been  opened  and  a  careful  and  thorough 
inspection  of  all  the  columns  made;  not  of  a  few  isolated  spots 
on  a  column  here  and  there,  but  of  the  entire  height  of  all  four 
sides  of  each  column. 

Undoubtedly,  there  are  grades  of  efficiency  in  concrete  work 
as  elsewhere,  although  the  best  is  none  too  good  in  most  cases. 
It  is  however  confidently  believed  that  serious  failures  of  rein- 
forced concrete  buildings  will  not  occur,  if  the  following  simple 
precautions  are  taken: 

Tie  all  steel  bars  into  the  next  span.  Use  closely  spaced  hoops 
in  all  columns.  And  see  that  the  concrete  is  hard  before  the  shores 
are  removed. 


CHAPTER  XVII 

SUPERINTENDENT'S  SPECIFICATIONS 

THE  following  instructions  have  been  used  by  the  Ransome  & 
Smith  Company,  as  a  standard  of  daily  practice  for  their  Super- 
intendents. 

General.  Order  and  close  attention  to  details  is  essential. 
Want  of  due  care  in  proportioning,  in  mixing,  or  in  the  placing 
of  the  steel  may  lead  to  destructive  results.  Reinforced  con- 
crete construction  requires  close,  continuous,  intelligent  super- 
vision. If  this  is  not  given,  disaster  is  not  far  off.  A 
superintendent  places  a  severe  handicap  upon  himself  unless 
he  so  organizes  his  men  that  from  the  lowest  up  to  the  high- 
est each  clearly  understands  his  duties  and  limitations  and 
knows  what  he  has  to  do,  and  unless  he  so  arranges  his  own 
time  that  he  can,  as  a  usual  thing,  devote  sufficient  time  to  the 
unexpected  demands  that  will  frequently  be  made  upon  him  for 
his  attention. 

All  accounts  must  be  kept  up  to  date  and  promptly  passed 
upon.  All  orders  must  pass  through  the  New  York  Office 
except  in  emergencies,  then  use  emergency  orders  and  forward 
copies  to  the  New  York  Office  for  confirmation. 

Temporary  Offices  and  Buildings,  Setting  up  Plant,  etc.  In 
setting  up  the  plant  see  that  the  mixer  is  in  good  line  and  securely 
placed  upon  a  level  bed.  Keep  all  running  gear  and  wearing 
parts  free  from  dirt  and  well  oiled  and  greased.  Keep  both 
inside  and  outside  of  the  mixer,  hoisting  tub,  hoppers,  gates, 
barrows,  etc.,  free  from  accumulated  dirt  or  concrete  of  over  a 
day  old.  Thoroughly  cleanse  off  every  night  the  day's  accumu- 
lation of  concrete  &nd  dirt  upon  tools  -and  machinery.  Protect 
scaffolds  and  all  openings  in  floors  with  suitable  hand-rails  and 
use  every  reasonable  precaution  against  accident. 

Excavating  and  Grading.  Make  these  of  the  dimensions  and 
depth  that  shall  be  determined,  upon  the  final  examination  of  the 

195 


196  REINFORCED  CONCRETE  BUILDINGS 

ground.  In  excavating,  give  sufficient  slope  to  the  sides  of  the 
hole  or  trench  to  prevent  caving  in,  or  protect  with  sheet  piling, 
and  excavate  the  final  depth,  corresponding  to  the  depth  of  the 
footing,  of  the  exact  size  required.  Do  not  excavate  this  final 
depth  much  before  the  time  for  filling  in  the  concrete.  Re-fill 
as  rapidly  as  the  work  permits  and  thoroughly  compact  all  re- 
filling that  subsequently  becomes  floor-bearing.  In  grading, 
follow  specifications  of  the  Contract. 

Molds.  Molds  shall  be  made  in  strict  accordance  with  draw- 
ings, which  will  be  furnished  from  headquarters. 

All  molds  to  be  thoroughly  fastened  together.  They  must 
not  only  be  set  true  and  plumb  to  line,  but  must  be  so  rigidly 
held  in  place  that  they  will  resist  successfully  all  tendency  to 
move  them  that  the  placing  of  the  concrete  may  give.  All 
interior,  or  molding,  faces  to  be  thoroughly  greased  with  crude 
oil  before  using,  and  thoroughly  cleaned  and  re-greased  at  every 
re-use.  All  open  joints,  broken  off  corners,  knot  holes,  to  be 
properly  puttied  up  with  ordinary  or  improved  putty  immediately 
before  placing  the  concrete. 

Concrete.     Every  car  load  of  cement  must  be  tested. 

Aggregates  will  be  finally  determined  upon,  at  which  time  the 
proportions  of  cement  with  these  will  be  given.  Salt  shall 
be  used  at  the  rate  of  four  pounds  to  a  barrel  of  cement. 
It  shall  first  be  dissolved  (in  a  tank  placed  above  the  level  of 
the  top  of  the  mixer)  to  a  saturated  solution.  Then  for  every 
bag  of  cement  used  in  the  batch,  add  three  pints  of  this  saturated 
solution. 

In  mixing,  put  the  water  in  first,  then  the  rock,  then  cement 
and  sand;  mix  thoroughly  and  in  placing  see  that  the  mixed 
concrete  is  of  such  consistency  and  character  that  it  will  pour 
from  the  wheelbarrows. 

In  starting  the  piers,  use  a  very  wet  concrete,  into  each  batch 
of  which  an  additional  bag  of  cement  has  been  placed,  for  the 
first  foot  of  height  of  the  piers.  Fill  each  pier  in  a  continuous 
operation  until  it  is  full ;  short  intermissions  of  time  not  sufficient 
to  permit  the  concrete  to  stiffen  may  be  disregarded  and  con- 
sidered as  continuous  filling.  Keep  the  column  work  at  least 
twelve  hours  ahead  of  the  floor  work. 

All  floors  must  be  thoroughly  rolled  with  the  first,  second,  and 
third  roller,  beginning  with  the  lightest;  continue  rolling  until 


SUPERINTENDENT'S  SPECIFICATIONS  197 

the  effect  thereof  is  not  apparent.1  The  concrete  shall  be  com- 
pleted in  any  one  unit  part  before  the  initial  set  appears  on  its 
surface. 

In  concreting  strike  the  bars  in  preference  to  the  concrete 
between  the  bars,  with  the  tampers.  Great  care  must  be  taken 
to  see  that  the  bars  are  thoroughly  embedded  in  the  concrete. 
Wherever  there  is  a  nest  of  cross-bars  that  the  concrete  will  not 
readily  penetrate,  pour  into  same  sufficient  cement  grout  1 : 2  to 
thoroughly  fill  all  spaces.  Special  care  must  be  taken  (especially 
in  hot  weather)  to  follow  up  this  grout  with  the  body  of  the 
concrete  before  the  grout  has  stiffened.  If  the  circumstances  are 
such  that  the  grout  stiffens  too  quickly  for  convenient  working, 
time  may  be  gained  by  throwing  on  the  face  of  the  grout  sufficient 
fresh  concrete  to  cover  it,  and  in  turn  should  this  fresh  concrete 
stiffen  before  it  is  covered  with  the  main  body  of  concrete,  it 
may  be  renewed  from  time  to  time  as  above  by  further  small 
additions  of  concrete.  It  is,  however,  important  that  neither 
the  original  surface  nor  any  of  the  renewed  surfaces  be  allowed 
to  stiffen  before  the  next  layer  is  applied. 

The  natural  slope  of  the  concrete  may  be  used  to  terminate 
any  days'  work  or  the  work  of  any  period  provided  the  following 
precautions  are  taken: — 

The  surface  of  this  slope  must  be  finished  with  a  drier  mixture 
than  usual  into  which  an  extra  batch  of  cement  has  been  added. 
Care  must  be  taken  also  that  this  sloping  surface  is  thoroughly 
tamped  down  into  a  compact  surface,  no  loose  porous  lumps  or 
portions  being  left  anywhere.  Before  starting  the  work  anew, 
if  this  concrete  is  sufficiently  soft  to  permit  of  the  cement  on  its 
surface  being  thoroughly  brushed  off  with  wire  brushes,  brush 
it  off  thus  and  top  off  the  surface  with  a  liberal  coat  of  pure 
cement  grout  well  brushed  in.  If  it  is  too  hard  for  this  oper- 
ation use  acid  joint.2  For  the  concrete  needed  to  cover  the 
sloping  surfaces  of  the  previous  work  throw  into  each  batch  an 

1  The  use  of  rollers  on  concrete  floors  is  not  in  accordance  with  usual  or 
current  practice.     However,  it  might  well  be  used  with  beneficial  results  as 
shown  by  my  own  practice  of  many  years.     Note  however  the  necessity  of 
good  strong  centering  that  will  not  yield  the  least  under  the  heaviest  roller 
used.  —  E.  L.  R. 

2  In  more  recent  practice,  the  vertical  joint  has  been  used,  as  the  sloping 
joint  is  rather  difficult  to  make  and  not  so  easily  repaired  in  case  of  trouble. 
—  A.  S. 


198  REINFORCED  CONCRETE  BUILDINGS 

extra  bag  of  cement,  then  proceed  with  the  work  as  previously 
described. 

Care  must  be  taken  to  leave  the  surface  of  the  concrete  at  the 
proper  level.  A  variation  of  more  than  1/4  "  in  the  finished  level 
will  not  be  considered  as  good  work. 

The  concrete  must  be  kept  wet  for  at  least  ten  days.  During 
concreting,  a  surveyor  must  be  kept  constantly  at  the  work  to 
determine  whether  or  not  there  is  any  settlement  in  the  falsework, 
and,  in  case  there  should  be  in  exceptional  cases,  the  defect  should 
be  rectified  before  the  concrete  sets.  This  also  applies  to  the 
alignment  of  the  exterior  surfaces  of  the  work. 

Steel.  All  steel  shall  be  kept  as  free  from  rust  as  prac- 
ticable. All  bars  must  be  placed  as  shown  on  the  drawings. 
No  variation  in  height  of  over  1/2 "  is  allowable,  or  in  other 
dimensions  of  over  3/4 ".  No  steel  must  appear  on  the  surface 
of  the  work.  Steel  that  would  otherwise  reach  the  surface 
must  be  wrapped  with  one  or  more  turns  of  protected  wire  or 
stout  marlin. 

All  the  steel  must  be  placed  ahead  of  the  concrete  except  where 
instructions  are  given  to  the  contrary  (in  very  exceptional  cases 
only). 

Finishing.  All  floors  shall  be  treated  with  acid  joint 
and  finish,  except  where  the  finish  is  put  on  before  the  floor  is 
thoroughly  set.  This  latter  shall  be  of  the  proportion  given, 
mixed  quite  stiff  and  thoroughly  well  troweled  down  and 
worked  to  a  true  smooth  finish.  Extreme  care  must  be  taken 
to  follow  closely  the  instructions  given  here  below  relative  to 
the  acid  joint.1 

Acid  Joint.  (1)  Thoroughly  sweep  the  floor,  removing  all 
loose  concrete  dust  and  debris,  etc. 

(2)  Wash  floor  thoroughly  with  water. 

(3)  Wash  floor  with  acid  mixture  (1  acid  18%  to  1  water) 

pouring  it  on  the  floor  freely  and  slowly  sweeping  it 
forward.  Follow  this  washing  with  a  second  and  third 
in  like  manner. 

(4)  Give   the  floor  a  final  and  thorough  washing  of  water. 

Immediately  before  laying  the  finish: — 

(5)  Thoroughly  wet  the  floor. 

iThis  method  is  covered  by  my  U.  S.  Patent  No.  860,942,  Oct.  3,  1905, 
—  Ernest  L.  Ransome. 


SUPERINTENDENTS  SPECIFICATIONS  199 

(6)  Rub  in  a  pure  cement  cream  with  wire  brushes,  sweep- 

ing forth  and  back,  going  over  the  same  ground  seven 
times. 

(7)  Before  this  shows  signs  of  setting,  sweep  over  it  more  of 

the  cement  cream  so  as  to  leave  on  the  surface  a  thick- 
ness of  about  1  /8  ".  This  cream  should  be  thicker  than 
the  first. 

(8)  Before  the  above  layer  shows  signs  of  setting,  put  on  the 

finish. 


CHAPTER   XVIII 
THE   ENGINEER 

As  compared  with  other  methods  of  construction,  reinforced 
concrete  is  essentially  a  manufacture.  From  the  earliest  days 
of  the  art,  this  was  recognized  by  the  makers,  who  called  them- 
selves artificial  stone  manufacturers  and  concrete  manufacturers. 
The  contractor  receives  the  raw  materials  in  the  form  of  cement, 
sand,  stone,  and  steel  bars,  and  from  these  he  manufactures 
the  structure,  while  in  other  types  of  building  contracting,  the 
finished  product  is  received  at  the  building,  and  then  simply 
erected  in  place.  Hence  the  reinforced  concrete  contractor  is 
charged  with  two  duties,  namely,  manufacture  and  erection, 
where  the  other  contractor  has  only  one,  namely,  erection. 

It  follows  that  expert  knowledge,  similar  to  that  possessed, 
for  instance,  by  the  steel  mill  organization,  must  in  some  manner 
be  supplied  on  the  reinforced  concrete  job.  According  to  cir- 
cumstances, the  expert  services  are  provided  by  either  the 
owner,  the  architect,  the  contractor,  or  the  local  building  depart- 
ment, if  indeed  they  are  not  wholly  absent,  which  appears  to 
happen  occasionally.  The  latter  case  is  entirely  too  frequent, 
due  to  the  prevailing  lack  of  understanding  of  the  difficulties 
incidental  to  reinforced  concrete  work.  It  is  the  duty  of  those 
who  know,  to  emphasize  this  fact,  each  in  his  own  locality,  so 
that  the  general  public  may  at  last  appreciate  the  absolute  neces- 
sity of  expert  skill  on  all  reinforced  concrete  work. 

It  is  not  believed  that  building  ordinances  or  regulations 
can  cope  with  this  problem  successfully.  In  Cleveland,  Ohio, 
the  owner  is  required  by  law  to  provide  an  inspector  who  shall 
be  present  at  all  times  when  concrete  is  being  placed  on  reinforced 
concrete  buildings;  the  inspector  must  pass  an  examination 
before  the  building  authorities.  But  this  examination  is  so 
elementary  that  nothing  even  remotely  approaching  expert 
supervision  is  obtained.  In  many  respects,  the  ordinance  is 
objectionable  to  the  owner,  who  cannot  always  command  the 

200 


THE  ENGINEER  201 

services  of  an  examined  inspector  at  the  proper  moment,  as 
well  as  to  the  contractor,  who  may  sometimes  have  to 
wait  for  the  inspector.  In  spite  of  these  minor  objections, 
the  system  is  undoubtedly  beneficial  in  Cleveland  at  the  pres- 
ent time,  although  the  possibilities  for  misuse  are  great  and 
always  present.  The  chief  objection  would  seem  to  be  in  the 
fact  that  the  owner's  conscience  is  lulled  to  sleep  in  the  hope 
that  a  paternal  city  department  will  see  him  through  all  troubles, 
while  as  a  matter  of  fact  the  inspection  is  barely  sufficient  to 
guard  against  gross  and  continuous  blunders. 

In  Boston,  Mass.,1  the  law  provides:  "When  the  struc- 
tural use  of  concrete  is  proposed,  a  specification  stating  the 
quality  and  proportions  of  materials  and  the  methods  of  mixing 
the  same  shall  be  submitted  to  the  Building  Commissioner, 
who  may  issue  a  permit  at  his  discretion  and  under  such  further 
conditions  in  addition  to  those  stated  below  as  he  sees  fit  to 
impose."  The  " conditions  stated  below"  give  the  allowable 
unit  stresses  and  other  provisions  foreign  to  our  present  purpose; 
the  discretionary  conditions  which  the  Commissioner  imposes 
at  the  present  time  are : 

(1)  That  the  plans  before  being  submitted  to  him  shall  have 
been  approved  by  an  expert  engineer  satisfactory  to  himself. 

(2)  That  during  the  placing  of  concrete  an  inspector  shall 
be  employed  at  the  expense  of  the  owner;  the  inspector  must 
be  satisfactory  to  the  Commissioner,  and  must  report  to  the 
Department  of  Buildings. 

In  regard  to  the  expert  engineers,  the  Commissioner  reserves 
to  himself  the  right  to  pass  upon  them  at  any  time.  Objec- 
tions have  been  raised  to  this  arrangement  on  the  ground  that 
the  expense  of  examining  the  plans  should  be  borne  by  the  Build- 
ing Department,  and  not  by  the  owner  (although  it  seems 
proper  that  each  owner  should  pay  the  expenses  of  his  own  plans). 
The  advisability  of  employing  an  expert  in  the  department  has 
been  considered,  but  so  far  without  result. 

In  regard  to  the  compulsory  inspection,  the  same  objec- 
tions may  be  raised  as  in  Cleveland,  that  really  efficient  inspec- 
tion is  not  obtained  in  that  manner,  and  that  the  owner  meanwhile 
is  brought  to  believe  that  his  work  is  efficiently  inspected. 

1  The  authors  are  indebted  to  Mr.  J.  R.  Worcester,  M.  Am.  Soc.  C.  E., 
for  information  in  regard  to  the  Building  Regulations  in  force  in  Boston. 


202  REINFORCED  CONCRETE  BUILDINGS 

The  Building  Regulations  of  Boston  and  Cleveland,  just 
cited,  throw  a  very  remarkable  light  upon  prevailing  conditions. 
It  is  almost  unbelievable  that  it  should  be  necessary  to  actu- 
ally force  the  owner  into  engaging  adequately  trained  men  to 
plan  and  supervise  the  structure  in  which  the  owner,  more  than 
any  one  else,  is  vitally  interested.  Undoubtedly,  the  efforts 
of  local  building  departments  have  succeeded  in  keeping  the 
standards  of  workmanship  and  design  above  a  certain  level, 
even  if  that  be  low,  but  it  must  not  be  forgotten  that  the  final 
decision  rests  with  the  public  generally  and  the  building  owners 
in  particular,  and  for  that  reason,  the  real  problem  before  the 
concrete  engineer  today  is  to  reach  and  educate  the  public  so 
that  better  work  is  not  only  insisted  upon  but  also  paid  for. 

It  would  be  very  desirable  if  uniform  regulations  could 
be  made  for  methods  of  design  and  calculation,  eventually  in 
the  form  of  State  Laws.  Efforts  toward  standardization  of 
calculations  have  been  made  by  the  American  Society  of  Civil 
Engineers  and  others,  and  that  such  recommendations  or  regu- 
lations are  not  impractical  may  be  seen  from  their  successful 
operation  in  Prussia,  Austria,  France,  etc.  Owing,  however, 
to  the  great  variation  in  available  supplies  of  aggregate,  the 
allowable  stresses  must  always  remain  a  local  issue. 

Various  influences  are  at  work  which  greatly  retard  the 
development  of  sound  engineering.  Certain  concerns  engaged 
in  the  selling  of  reinforcement  will  furnish  free  plans  showing 
designs  calculated  to  land  the  job  rather  than  to  give  efficient 
service.  The  method  is  objectionable  when  worked  through 
the  medium  of  a  small  contractor,  but  much  more  so  when  a 
so-called  " architect"  is  made  to  act  as  a  cat's-paw.  The 
architect  (or  engineer)  who  holds  himself  out  as  qualified  to 
design  reinforced  concrete  work,  and  either  has  not,  or  does  not 
provide  for  the  requisite  skill,  is  guilty  of  deception,  and  obtains 
his  money  under  false  pretenses.  As  a  matter  of  fact,  all  our 
best  architects  have  competent  engineers  on  their  staff,  or 
engage  the  necessary  talent  when  required,  and  the  owner  can 
always  obtain  such  services  by  simply  insisting  upon  having 
them. 

Another  objectionable  practice  has  sprung  from  the  indis- 
criminate use  of  ''tables  of  design."  The  modern  steel  indus- 
try would  certainly  be  an  impossibility  without  standard  shapes, 


THE  ENGINEER  203 

and  here  the  structural  steel  tables  in  common  use  are  the  only 
proper  thing.  But  while  a  certain  degree  of  standardization 
in  reinforced  concrete  construction  is  urgently  important,  it 
is  practically  impossible  to  provide  for  the  innumerable  pos- 
sibilities of  design,  at  least  at  present.  Moreover,  while  the 
table  itself  may  be  a  labor-saving  device,  it  is  likely  to  be  used 
most  by  those  who  are  least  conversant  with  the  underlying 
principles,  leading  to  disastrous  results. 

Again  in  certain  sections,  particularly  in  the  Middle  West, 
a  class  of  contractors  has  been  created  whose  slogan  appears 
to  be:  "get  the  job  at  any  cost."  No  contractor  can  afford  in 
a  lump  sum  contract  to  take  work  at  less  than  actual  cost  to 
him  plus  a  reasonable  profit,  the  cost  to  include  overhead  ex- 
penses, depreciation,  idleness  of  plant  and  staff,  contingencies, 
etc.  For  a  while,  a  contracting  business  may  be  run  in  viola- 
tion of  these  principles,  but  not  for  long.  Of  course,  every  job 
on  which  the  specifications  are  honestly  and  consistently  en- 
forced hastens  the  day  of  judgment  for  such  concerns,  and  they 
strongly  resent  anything  that  looks  like  supervision.  These 
concerns  have  injured  not  only  themselves,  but  have  succeeded 
in  lowering  the  general  standard  of  workmanship  by  training 
foremen  and  young  engineers  in  sloppy  and  slovenly  work. 
When  these  abuses  become  too  great  emergency  provisions 
are  in  order,  and  the  compulsory  inspection  paid  for  by  the 
owner  under  the  supervision  of  the  building  department  is  one 
way. 

At  the  present  time,  there  are  reasons  for  believing  that 
reinforced  concrete  contracts  should  be  let  on  the  "cost  plus 
profit"  basis.  Such  contracts  protect  the  owner  against  pooled 
bids  and  against  extortionate  charges  for  contingencies  or  profits. 
It  is  evident  that  the  structural  steel  contractor  has  no  "con- 
tingencies of  manufacture,"  but  only  "contingencies  of  erec- 
tion," while  the  reinforced  concrete  contractor  has  both. 

In  fact,  if  the  contract  be  not  awarded  to  the  lowest  bidder, 
there  is  no  good  reason  for  taking  bids,  and  if  the  owner  has 
so  much  confidence  in  any  one  bidder  that  he  prefers  him  in 
spite  of  his  higher  bid,  he  might  as  well  trust  him  to  the  extent 
of  giving  him  the  contract  on  the  cost  plus  profit  basis.  We 
are  not  here  concerned  in  discussing  the  various  types  of  con- 
tracts possible  under  this  system,  as  to  whether  the  maximum 


204  REINFORCED  CONCRETE  BUILDINGS 

cost  'ought  to  be  guaranteed  or  not,  or  whether  the  profit  should 
be  a  percentage  of  the  actual  cost  or  a  certain  stipulated  sum. 

It  is  believed  that  such  contracts  are  usually  given  to  con- 
tractors having  an  engineering  department  in  their  organiza- 
tion, and  who  are,  as  a  matter  of  fact,  "contracting  engineers" 
whether  so  called  or  not.  We  must  remember  that  reinforced 
concrete  construction  was  first  introduced,  and  has  since  mainly 
been  developed,  by  just  such  men  or  concerns.  It  is  safe  to 
say  that  a  very  large  portion,  if  not  the  largest  portion,  of  all 
reinforced  concrete  buildings  of  any  consequence  is  erected 
by  "  constructing  engineers,"  who  plan,  design,  and  erect  the 
work  from  start  to  finish,  frequently  on  the  cost  plus  profit 
basis.  The  owner  should  have  these  plans  checked  by  a  con- 
sulting engineer  and  provide  for  adequate  inspection  of  the  work. 
The  position  of  the  inspecting  engineer  is  one  that  calls  for 
considerable  tact,  because  he,  as  well  as  the  contractor,  are 
virtually  members  of  the  same  organization,  viz.,  of  the  owners' 
building  staff. 

Under  the  lump  sum  contract,  the  engineer's  position  is 
radically  different.  He,  and  he  alone,  should  prepare  the  gen- 
eral and  detail  plans,  with  adequate  specifications,  and  once  the 
contract  is  let,  it  becomes  his  duty  to  enforce  the  specifications 
in  letter  and  spirit,  making  himself  as  disagreeable  as  condi- 
tions demand.  Even  if  the  specifications  (or  contract)  give  the 
engineer  the  right  to  make  necessary  alterations,  he  should  be 
exceedingly  careful  not  to  waive  any  of  the  requirements  by 
commission  or  omission.  Inspection  of  this  kind  is  efficient 
only  when  explicit  and  full  specifications  have  been  prepared, 
but  this  does  not  mean  that  the  specifications  should  be  burden- 
some or  unfair  to  the  contractor.  It  is  no  easy  matter  to  write 
good  specifications  for  reinforced  concrete  work,  and  it  requires 
first  of  all  full  acquaintance  with  local  conditions.  There  are 
many  places,  for  instance,  where  " clean,"  " sharp"  sand  can- 
not be  obtained  locally,  and  the  engineer  must  so  word  his 
specifications  that  suitable  sand  is  called  for,  and  he  must  then 
see  that  good  sand  is  really  used;  —  not,  as  some  engineers  do, 
call  for  clean,  sharp  sand,  and  then  allow  the  use  of  sand  that  is 
neither  the  one  nor  the  other. 

Attention  is  called  to  the  difficulties  encountered  in  making 
monthly  estimates  for  reinforced  concrete  buildings.  The  false- 


THE  ENGINEER  205 

work  enters  only  as  machinery,  tools,  or  other  appliances,  and 
its  full  value  should  not  at  any  time  enter  into  the  estimate, 
but  only  a  certain  proportion  of  its  value.  This  leaves  consider- 
able room  for  argument  as  to  just  what  proportion  to  include; 
the  better  and  safer  way  is  to  have  a  clause  in  the  contract 
stating  that  a  certain  reasonable  proportional  amount  of  the 
contract  price  must  be  paid:  (1)  when  the  footings  are  in;  (2) 
when  the  first  floor  has  been  concreted,  etc.,  etc.  It  is  very 
much  easier  to  arrange  the  amounts  to  be  paid  before  the  con- 
tract is  signed  than  after  the  work  is  under  way. 

It  is  now  evident  that  whatever  the  position  of  the  engineer, 
-  whether  connected  with  owner,  architect,  or  contractor,  — 
he  must  possess  certain  qualifications  of  his  own.  First  of  all, 
he  must  make  himself  felt  as  a  useful  factor  in  the  community, 
and  not  be  satisfied  with  remaining  a  subordinate,  and  apparently 
superfluous  appendix.  He  alone  has  it  in  his  hands  to  make  the 
industry  advance  or  decline,  and  his  essential  function  is,  not 
only  to  economize  in  the  proper  place,  but  to  make  the  owners 
see  the  folly  of  parsimony.  He  will  have  to  overcome  criticisms 
of  impracticability  and  extravagance,  and  this  will  be  the  more 
difficult  as  he  will  rarely  be  brought  face  to  face  with  the  charges. 

He  must  be  an  expert  designer,  not  only  of  the  usual  ribbed 
floors,  arranged  in  the  conventional  cigar-box  type  of  factory 
building,  but  also  of  the  more  complicated  types  of  flat  floors, 
of  ribbed  arches  and  other  unusual  forms  for  which  reinforced 
concrete  is  so  well  adapted  and  as  yet  so  little  used.  Never- 
theless his  ability  as  a  mathematician  must  not  kill  his  ability 
as  a  business  man,  for  if  he  cannot  get  the  work  to  exercise  his 
mathematics  on,  he  will  have  scant  use  for  them.  He  must  be 
fully  posted  on  methods  of  erection  —  not  only  how  to  do  things, 
but  also  how  not  to  do  them  —  yet  his  knowledge  must  not  make 
him  overbearing  with  the  common  foreman  who  "knows  every- 
thing about  it,"  yet  whose  main  asset  is  his  ignorance. 

Granting  now  that  our  engineer  approaches  to  some  extent 
the  ideal  just  outlined,  he  must  also  possess  a  certain  amount 
of  skepticism  in  regard  to  precedents.  Without  question,  there 
are  wide  fields  for  investigation  as  yet  open.  We  have  referred 
in  an  earlier  chapter  to  the  fallacy  of  too  implicit  faith  in  cement 
testing.  We  have  considered  the  impossibility  of  current  ideas 
of  shear  in  reinforced  concrete  beams.  There  may  be,  and  prob- 


206  REINFORCED  CONCRETE  BUILDINGS 

ably  are,  many  others.  Criticism  of  this  kind  is  beneficial, 
not  only  professionally,  but  sometimes  financially  as  well,  be- 
cause sound  criticism  leads  to  improvements,  and  good  improve- 
ments are  well  worth  while. 

In  order  to  gain  material  benefit  from  an  improvement  or 
invention,  it  must  be  patented.  Reinforced  concrete  men  are 
too  prone  to  decry  the  value  of  patents  generally,  but  this  atti- 
tude appears  to  be  founded  in  ignorance.  In  order  to  avoid 
infringement,  the  engineer  must  certainly  be  familiar  with 
patents  and  patent  law;  only  in  that  way  he  can  save  the  client 
undue  expense  and  trouble,  and  judge  for  himself  of  the  value 
of  a  new  invention.  The  only  circumstance  saving  many  a 
man  from  patent  suits  is  that  the  patentee  cannot  afford  the 
expenses  of  court  trial,  which  may  run  anywhere  from  $5,000 
to  $20,000  or  more,  and  extend  over  many  years.  If  there  are 
any  good  and  substantial  reasons  for  granting  patents,  the 
engineering  profession  should  recognize  the  existing  conditions 
and  inform  themselves,  treating  patent  rights  in  the  same  man- 
ner as  they  do  other  property;  if  no  such  reasons  exist,  the 
engineers  should  use  their  influence  in  having  the  patent  office 
abolished.  There  is  little  likelihood  that  the  latter  alternative 
will  be  followed,  and  patents  should  therefore  be  respected. 
One  way  of  ensuring  the  rights  of  the  patentee  would  be  to  have 
an  injunction  issued  at  once  when  proper  evidence  was  pre- 
sented to  the  court,  and  leave  it  for  the  infringer  to  prove  the 
patent  invalid;  as  it  is,  the  patentee  practically  has  to  prove  the 
validity  of  his  patent  before  any  injunction  will  be  issued.  It 
would  well  pay  the  owner  to  see  that  his  engineer  is  well  posted 
on  the  question  of  patent  rights,  for  if  infringement  should 
occur,  the  patentee  will  certainly  look  to  the  owner  for  reparation. 


CHAPTER   XIX 
THE   THEORY  OF  BEAMS  AS  ILLUSTRATED  BY   TESTS 

The  Extensibility  of  Concrete  is  not  changed  by  the  presence 
of  reinforcement.  It  was  discovered  in  tests  made  at  the  Uni- 
versity of  Wisconsin  in  1901-1903  that  beams  cured  in  water 
and  partially  dried  showed  " watermarks"  or  fine  dark  lines 
on  the  tension  side  under  loads  which  would  have  fractured 
non-reinforced  pieces,  and  it  was  proved  that  these  watermarks 
indicate  cracks.1 

In  reinforced  concrete  beams,  these  cracks  appear  under 
tension  stresses  in  the  steel  of  about  5,000  Ibs.  per  square  inch, 
and  we  are  therefore  not  justified  in  calculating  on  any  tensile 
resistance  in  the  concrete. 

The  Shear  Resistance  of  Concrete  is  not  affected  by  the  pres- 
ence of  reinforcement.  Prof.  Morsch2  made  shear  experiments 
with  cement  mortar  prisms  1"  x  7"  in  section  and  found: 

for  mixture  1:3;  2  years  old:  879,835,  1098  Ibs./sq.  inch 

average  937  Ibs./sq.  inch 

for  mixture  1:4;  6  weeks  old:  549,593,  441  Ibs./sq.  inch 

average  528  Ibs./sq.  inch 

Reinforced  prisms  of  same  mixture,  size,  and  age  as  the  last 
series  sheared  under  the  following  stresses:  550,  495,  528,  451, 
473  Ibs.  per  square  inch.  It  made  little  difference  whether  the 
reinforcement  was  straight  or  bent.  The  final  carrying  capac- 
ity of  the.  reinforced  prisms  was,  however,  much  greater  than 
their  apparent  shear  resistance,  for  after  the  concrete  had  sheared 
it  was  still  possible  to  increase  the  loads  considerably.  Professor 
Morsch  considers  that  this  increase  was  due  to  the  shear  resist- 
ance of  the  steel,  which,  mathematically  speaking,  was  stressed 
in  shear  as  follows,  when  the  final  collapse  took  place: 

1  Turneaure  and  Maurer:  Principles  of  Reinforced  Concrete  Construc- 
tion, 2d  Edition,  p.  42. 

Subsequent  tests  by  Bach  (Zeitschrift  des  Vereines  deutscher  Inge- 
nieure,  Band  51,  Nr.  26)  have  fully  supported  the  Wisconsin  tests. 

2  Morsch:   Concrete  Steel  Construction,  p.  33  ff. 

207 


208    '  REINFORCED  CONCRETE  BUILDINGS 

Specimen  1:  47,650  lbs./sq.  in.  straight  bars  only 
"  2:  45,230  "  "  "  "  "  " 

3:55,050    "     "     "       straight  and  bent  bars 

4:  47,080  "  "  "  "  "  " 

"  5:  50,350  "  "  "  "  "  "  " 

The  ultimate  shear  strength  of  the  steel  was  only  47,790  Ibs.  per 
square  inch,  so  where  specimen  3  acquired  its  additional  17  per 
cent,  strength  does  not  seem  clear.  Specimen  2  failed  under  a 
load  of  40  tons,  "at  which  point  a  horizontal  crack  appeared  at 
the  left  end."  This,  we  know,  is  an  indication  that  the  steel 
is  pulling  out  of  the  concrete,  and  it  seems  altogether  likely 
that  the  resistance  really  measured  in  these  specimens  was  the 
tensile  resistance  of  the  reinforcement,  in  accordance  with 
the  theories  advanced  in  Part  II,  Art.  57,  of  this  book. 

Various  other  tests  have  been  made  to  determine  the  resist- 
ance of  concrete  to  pure  shear.  They  generally  confirm  the 
figures  given  directly  above,  but  the  results  vary  greatly  owing 
to  the  great  difficulty  in  eliminating  tensional  stresses.  In 
practical  construction,  pure  shear  is  rarely  encountered  in  rein- 
forced concrete  beams. 

The  Function  of  the  U-Bars.  With  the  foregoing  remarks  in 
mind  we  must  admit  that  the  U-bars  cannot  in  any  way  influ- 
ence the  shear  resistance  of  the  concrete.  If  we  consider  the 
U-bars  as  active  in  shear,  their  action  cannot  take  place  before 
the  shear  resistance  of  the  concrete  is  exhausted,  and  whatever 
view  we  take  of  the  stresses,  the  total  shear  resistance  of  the 
beam  is  not  the  sum  of  that  of  the  concrete,  and  that  of  the  U- 
bars  (or  other  "shear"  reinforcement).  In  this  book,  the 
U-bars  have  been  considered  as  (1)  retarding  the  sliding  of 
the  main  tension  reinforcement  and  (2)  supplying  the  vertical 
tension  resistance  caused  by  deviation  from  the  equilibrium 
curve  of  either  the  compression  or  the  tension  "chords."  The 
first  proposition  is  easily  investigated  by  test;  the  second  is 
closely  related  to  problems  connected  with  trussed  rods  and 
kindred  matters,  and  will  be  considered  in  that  connection  here 
below. 

The  Stirrups  Retard  the  Sliding  of  the  main  tension  rods. 
The  "Commission  du  Ciment  Arme"  (1907)  tested  specimens 
as  shown  in  Figure  148  a,  6,  and  c.  The  specimens  gave  the  fol- 
lowing average  sliding  resistance  per  sq.  inch  of  embedded  sur- 


THE  THEORY  OF  BEAMS  AS  ILLUSTRATED  BY  TESTS     209 

face,  the  first  group  having  stirrups  of  flat  iron,  the   second  of 
round  iron: 


6  months  old 

3  months  old 

a 
b 
c 

159  Ib./sq.  inch 
214  Ib./sq.  inch 
281  Ib./sq.  inch 

125  Ib./sq.  inch 
252  Ib./sq.  inch 
284  Ib./sq.  inch 

or  about  the  same  values  for  the  specimens  with  stirrups  as  for 
rods  centrally  embedded  in  a  block  of  concrete.  This  shows 
the  increasing  importance  of  U-bars  in  beams  with  thin  con- 
crete covering  on  the  rods,  even  in  the  case  where  U-bars  are 
not  theoretically  required. 


FIGURE  148. 


These  tests  show  the  gripping  action  exerted  by  the  U-bar 
on  the  rod,  and  explain  in  part  the  tendency  of  the  beams  to 
crack  at  the  U-bars,  because  the  U-bars  act  as  washers  on  the 
rod,  so  that  the  concrete  naturally  would  split  immediately 
behind  such  points. 

Straight  Reinforcement  in  T-Beams  —  German  Tests.  In 
the  famous  series  of  T-beams  tested  by  Prof.  Morsch,  beams 
I,  II,  and  III  were  equipped  with  U-bars  for  one-half  the  length 
only.  The  beams  all  failed  at  the  end  without  U-bars  under 
loads  fairly  proportional  with  the  width  of  the  stem,  showing 
that  the  resistance  of  the  stem  was  the  deciding  factor.  It 
makes  little  difference  for  our  present  purpose  whether  the  fail- 
ure was  caused  by  actual  shear  or  by  the  pulling  out  of  the  rein- 
forcement ;  —  Prof.  Morsch  appears  to  favor  the  former  of  these 
alternatives,  although  in  the  two  beams  with  narrow  stems,  the 
concrete  surrounding  the  reinforcement  was  split  off,  indicat- 


210  REINFORCED  CONCRETE  BUILDINGS 

ing  that  the  concrete  was  not  sufficient  to  resist  the  lateral  expan- 
sion, thus  allowing  the  rods  to  slip.  The  resistance  called  into 
action  in  this  manner  would  be  proportional  with  the  thickness 
of  the  stem. 

These  tests  show  conclusively  that  T-beams  with  straight 
reinforcement  only  and  without  U-bars  are  not  economical  struc- 
tures. As  to  the  U-bars  themselves,  the  tests  show  they  are 
beneficial,  and  Prof.  Morsch  further  states: 

"  If  the  cause  and  the  formation  of  the  cracks  in  these  three 
beams  are  examined,  it  is  established  that  the  cracks  first  became 
visible  where  the  moment  was  greatest,  and  that  with  increase 
of  load  more  distant  cracks  appeared.  On  the  end  supplied  with 
stirrups,  the  cracks  appeared  to  occur  at  the  sections  in  which 
the  stirrups  were  located,  since  the  concrete  section  was  weak- 
ened at  those  points."  The  same  observation  has  been  made 
in  other  investigations. 

These  beams  were  tested  with  a  uniformly  distributed  load 
covering  the  entire  span. 

Bent  Reinforcement  in  T-Beams  —  German  Tests.  In  con- 
tinuation of  the  tests  just  described,  Prof.  Morsch  investigated 


Beam  of  the  TRAJECTORY  Type 


FIGURE  149. 


several  beams  with  a  combination  of  bent  and  straight  bars. 
Two  distinct  types  were  used,  the  bent  bars  being  of  either  the 
" trajectory"  type  (Figure  149)  or  of  the  "suspension"  type 
(Figure  150).  In  the  table  herewith,  the  principal  details  of 


Beam  of  the  SUSPENSION  Type 

FIGURE  150. 

the  arrangements  are  given  (the  letters  T  and  S  indicating  the 
type  of  bent  reinforcement),  as  well  as  the  ultimate  load. 


THE  THEORY  OF  BEAMS  AS  ILLUSTRATED  BY  TESTS     211 
TABULATED  RESULTS  OF  PROF.  MORSCH'S  BEAM  TESTS 


Tension  Rods 

I* 

O> 

Beam 

Type 

B 
No. 

ent 

Straight 

1\ 

fl  o 

•gs 

U-bars 

Type  of 
Loading 

"S-S    u    03 

111  I 

P>3  £3 

Diam. 
in 

No. 

Diam. 
in 

m/m 

m/m 

IS  -2 

H«2 

IV 

T 

3 

15  and  1 

18 

14 

none 

Uniform 

42.0 

VI 

T 

3 

15  and  1 

18 

14 

full  supply 

load 

37.8 

V 

S 

-2 

15  and  2     16 

14 

one  end  only 

covering 

31.0 

entire  span 

Two 

VII 

T 

3 

16  and  1 

16 

14 

full  supply 

concentrated 

34.0 

VIII 

S 

2 

16  and  2     16 

10 

one  end  only 

loads 

23.4 

IX 

S 

2 

16  and  2     16 

14 

one  end  only 

at  third 

25.6 

points 

X 

T 

3 

16  and  1 

16 

14 

one  end  only 

One 

27.0 

XI 

S 

2 

16  and  2     16 

14 

none 

concentrated 

26.0 

XII 

T 

3 

16  and  1 

16 

14 

none 

load 

26.0 

at  center 

The  tests  naturally  divide  themselves  into  three  groups, 
according  to  the  manner  of  loading: 

(1)  Uniform  load,  beams  IV,  VI,  and  V.  It  is  at  once 
apparent,  by  comparing  beam  IV  (without  U-bars)  with  beam 
VI  (with  U-bars),  that  the  influence  of  the  U-bars  is  very  slight, 
if  any,  the  difference  in  ultimate  load  being  accounted  for  by 
the  fact  that  the  ends  of  the  straight  bar  in  beam  IV  were  hooked, 
while  those  in  beam  VI  had  no  hooks.  Both  of  these  beams  were 
of  the  trajectory  type,  and  if  we  compare  them  with  beam  V  of 
the  suspension  type,  the  superiority  of  the  trajectory  type  seems 
clearly  established.  But  we  must  not  lose  sight  of  the  fact  that 
in  the  two  first  beams  three  of  the  four  rods  were  bent  up,  while 
in  the  latter,  only  two  of  the  four  rods  were  bent  up.  This 
beam  failed  in  the  end  without  U-bars,  and  while  therefore 
this  group  does  not  prove  the  author's  theories,  as  outlined  in  a 
preceding  chapter,  it  does  not  disprove  them,  and  still  leaves 
the  question  open  whether  or  not  the  bending  of  one  additional 
rod,  or  the  proper  use  of  U-bars,  would  not  have  changed  the 
results  materially.  It  will  be  remembered  that  in  a  T-beam, 
the  straight  reinforcement  is  effective  only  as  reinforcement  of 
the  stem,  while  the  bent  bars  correspond  to  the  flange;  if  the  rods 
are  not  so  arranged,  stirrups  must  be  introduced  to  again  bal- 


212  REINFORCED  CONCRETE  BUILDINGS 

ance  the  design,  the  size  of  the  U-bars  being  in  direct  ratio  to 
the  violation  of  the  principle  outlined. 

(2)  Two  concentrated  loads,  beams  VII,  VIII,  and  IX. 
Here  again,  beam  VII  of  the  trajectory  type,  with  a  full 

supply  of  U-bars,  is  compared  with  two  beams  of  the  suspension 
type,  the  two  latter  being  without  U-bars.  Again  the  traject- 
ory type  seems  superior  to  the  suspension  type,  and  again  we 
find  the  reason  to  be  that  in  the  trajectory  beam,  the  proper 
amount  of  rods  have  been  bent  up,  while  in  the  suspension  type, 
only  two  of  the  four  rods  have  been  bent,  and  no  U-bars  have 
been  introduced  to  overcome  the  deficiency. 

It  is  rather  interesting  to  note  that  the  difference  in  width 
of  stem  between  beam  VIII  (10  cm.)  and  beam  IX  (14  cm.)  affects 
the  ultimate  strength  but  slightly,  both  beams  being  of  the 
suspension  type. 

In  his  discussion  of  these  tests,  Prof.  Morsch  has  taken 
occasion  to  criticize  the  suspension,  or  Hennebique,  type.  It 
is  to  be  regretted  that  the  tests  were  not  carried  out  so  as  to 
have  the  same  number  of  bent-up  bars  in  both  of  the  types 
considered,  in  which  case  the  suspension  type  would  probably 
have  stood  up  as  well  as  the  trajectory  beams.  It  is  only  fair 
to  note  that  the  U-bars  or  stirrups  have  always  been  considered 
as  an  essential  part  of  the  Hennebique  system,  and  that  such 
tests  as  these,  however  valuable  otherwise,  give  no  indication 
whatever  as  to  the  merits  of  this  system. 

(3)  One  concentrated  load  at  center,  beams  X,  XI,  XII. 

In  this  group,  the  two  systems  give  the  same  carrying 
capacity,  owing  undoubtedly  to  the  fact  that  in  no  one  of  these 
beams  the  reinforcement  is  arranged  according  to  the  equilib- 
rium curve,  while  in  no  case  U-bars  have  been  introduced  to 
compensate  for  the  deviation. 

Bent  Bars  in  T-Sections — Author's  Tests.  The  beam  tests 
just  referred  to  were  published  by  Prof.  Morsch  in  "Deutsche 
Bauzeitung,"  April  13,  1907.  It  occurred  to  the  author  of  the 
theory  of  this  present  volume  that  the  description  of  the  action 
of  the  suspension  rods  was  subject  to  doubt,  for  the  reasons 
outlined  above,  and  that  additional  information  might  possibly 
be  gained  by  tests  on  beams  with  trussed  rods  only.  The  author 
designed  a  series  of  nine  test  beams  which  were  tested  in  the 
winter  1907-1908  at  Case  School  in  Cleveland,  in  co-operation 


THEORY  OF  BEAMS  AS  ILLUSTRATED  BY  TESTS     213 


Type  A 
ams  1,4,7 


n 


TypeB- 
ams  2,8. 


\ 


>>  c 

"I 


r  U 


omag  jo  Jl«H  9U0  I  5»" 

^— ^H^ 


|< 


214  REINFORCED  CONCRETE  BUILDINGS 

with  Prof.  F.  H.  Neff.  It  will  be  seen  from  Figure  151  that 
these  beams  had  no  straight  reinforcement,  and  that  the  sloping 
stem  terminated  at  the  supports,  so  as  to  make  the  system  one 
of  equilibrium  under  two  concentrated  loads.  The  results  were 
first  published  in  the  Engineering  Record,  August  22,  1908,  from 
which  the  following  is  an  extract: 

"  Three  different  molds  were  made,  types  A,  B,  and  C, 
respectively,  each  one  of  which  was  used  three  times  with  a 
different  percentage  of  steel  for  reinforcement  of  the  beam. 
In  this  way,  three  beams  of  type  A  were  made,  one  of  which  was 
reinforced  with  0.5  per  cent.,  one  with  0.75  per  cent.,  and  one 
with  1.0  per  cent.  In  the  same  way  three  beams  B  and  three 
beams  C  were  made,  reinforced  as  described,  so  that  of  the  total 
number  of  nine  beams  no  two  were  alike  in  all  respects,  but  any 
one  beam  would  have  a  corresponding  one  which  was  different  in 
one  detail  only.  In  this  way,  it  would  be  possible  to  compare  the 
beams  and  find  the  exact  effect  of  a  certain  change,  which  is  a 
safer  way  than  to  try  to  obtain  absolute  results  from  so  few  tests. 

"All  the  beam  swere  provided  with  U-bars  in  one  end  only, 
the  object  being  to  show  that  the  stirrups  were  of  no  conse- 
quence at  all.  The  stirrups  made  no  difference  in  the  results 
obtained,  four  of  the  nine  beams  failing  in  the  end  equipped 
with  U-bars. 

"Two  short  cross  bars  were  placed  in  the  slab  at  the  points 
where  the  loads  were  applied,  and  three  similar  bars  were  placed 
in  the  slab  near  the  support.  These  bars  were  i-inch  square 
twisted  bars.  The  main  tension  bars  were  1-inch  square  twisted 
Ransome  bars.  It  was  found  that  the  elastic  limit  of  these 
bars  averaged  about  56,000  Ibs.  per  square  inch,  and  their  ulti- 
mate breaking  strength  was  73,600  Ibs.  per  square  inch.- 

"  The  concrete  was  made  quite  wet  and  very  carefully  placed. 
The  mixture  used  was  1:2:3^,  Lake  Erie  sand  and  Euclid  bluestone 
being  used  for  the  aggregates.  The  strength  of  the  cubes  was  low, 
as  might  be  .expected  with  the  aggregates  used,  and  the  average 
of  the  6-inch  cubes  in  pounds  per  square  inch  was  as  follows: 

Age,  days 7  14  28  60 

Strength,  pounds 660  1,065  1,440          1,787 

"The  beams  were  all  tested  when  sixty  days  old.  In  the 
table  here  below  the  results  are  given,  and  this  table,  together 


THE  THEORY  OF  BEAMS  AS  ILLUSTRATED  BY  TESTS     215 


with  the  diagrams  of  the  beams,  should  give  all  the  information 
needed.  Attention  is  called  to  the  ways  of  supporting  beams  5 
and  6.  While  all  the  other  beams  are  supported  at  the  point 
where  the  sloping  stem  begins,  these  two  beams  are  supported 
further  out  from  the  stem,  making  the  overhang  shorter  for 
them  than  for  the  similar  beams  of  same  type. 

"As  to  the  column  headings  used  in  the  table,  the  percen- 
tage of  reinforcement  is  calculated  with  reference  to  the  '  enclos- 
ing rectangle'  proposed  by  Professor  Talbot.  Under  'lever'  the 
distance  from  point  of  support  to  point  of  application  of  the 
load  is  given,  while  'overhang'  means  the  length  of  the  pro- 
jecting end  beyond  the  support. 

"The  bending  moment  given  in  this  table  is  found  by  mul- 
tiplying the  'lever'  by  one-half  of  the  ultimate  load,  disre- 
garding entirely  the  weight  of  the  beam  itself.  The  lever  arm 
of  the  internal  stresses  is  assumed  to  be  0.85  times  the  distance 
from  the  top  fiber  to  the  center  of  the  steel,  which  distance  is 
approximately  9  in.,  giving  a  lever  arm  of  7.65  in.  This,  of 
course,  is  not  quite  correct,  as  the  position  of  the  neutral  axis 
varies  with  the  percentage  of  steel  and  the  coefficient  of  elas- 
ticity, which  latter  again  depends  upon  the  stress  on  the 
concrete.  It  is,  however,  sufficiently  accurate  considering  the  un- 
avoidable variations  in  the  position  of  the  steel  bars  and  in  the 
elastic  properties  of  the  concrete,  and  the  'total  stress  in  the 
steel'  may  therefore  be  found  by  dividing  the  bending  moment 
by  7.65,  giving  the  values  shown  in  the  table  as  well  as  the 
stress  in  the  steel  per  square  inch  of  its  cross-section. 

RESULTS  OF  TESTS  AT  CASE  SCHOOL. 

Beam.    Type.    Per  cent.     Lever.     Overhang.     Ultimate 

load. 


0.5 

0.5 

0.5 

0.75 

0.75 

0.75 

1.00 

1.00 

1.00 


30  in 
26  in 
22  in 
30  in 
30  in 
30  in 
30  in 
26  in 
22  in 


10  in. 
14  in. 
18  in. 
10  in. 
10  in. 
10  in. 
10  in. 
14  in. 
18  in. 


12,500 
16,000 
27,800 
11,900 
16,200 
16,000 
13,950 
22,000 
28,900 


Bending 

-  Stress  in  steel  

moment. 

Total. 

Per  sq.  in. 

187,500 

24,500 

49,000 

208,000 

27,200 

54,400 

305,800 

39,900 

79,800 

178,500 

23,300 

31,000 

243,000 

31,800 

42,400 

240,000 

31,400 

41,800 

209,250 

27,300 

27,300 

286,000 

37,400 

37,400 

317,900 

41,600 

41,600 

"Eef erring  now  to  the  several  photographs  of  the  beams 
after  failure,  it  will  be  noticed  that  the  failures  are  of  uniform 
nature.  Comparing  the  figures  given  in  the  table  above,  it 
will  be  seen  that  the  ultimate  load  varies  greatly  as  well  as  the 
total  stress  and  the  stress  per  square  inch.  If  the  failure  has 


216 


REINFORCED  CONCRETE  BUILDINGS 


Beam  1 


Beam  2 


Beam  4 


FIGURE  152. 


FIGURE  153. 


FIGURE  154. 


Beam  5  FIGURE  155. 

THE  CASE  SCHOOL  BEAMS  AFTER  TESTING. 


TtiE  THEORY  OF  BEAMS  AS  ILLUSTRATED  BY  TESTS     217 

a  common  cause  in  all  these  beams  it  cannot  be  due  to  either 
tension  or  compression  in  the  usual  sense  of  the  word.  It  may 
also  be  assumed  that  shear  had  little  to  do  with  the  failure. 


Beam  6 


FIGURE  156. 


Beam  8 


FIGURE  157. 


Beam  9  FIGURE  158. 

THE  CASE  SCHOOL  BEAMS  AFTER  TESTING. 

On  account  of  the  trussed  form  of  the  beams,  the  steel  follows 
the  curve  of  equilibrium  of  the  external  forces  acting  upon  the 
beam,  and  the  only  stresses  possible  are  tension  in  the  steel  and 
compression  in  the  concrete. 


218  REINFORCED  CONCRETE  BUILDINGS 

"This  is  also  evident  from  the  behavior  of  the  beams  under 
load.  The  cracks  started  on  the  tension  side  and  opened  slowly 
with  increasing  load,  at  the  same  time  becoming  longer,  until 
finally  the  compressive  area  left  above  the  top  of  the  crack  became 
too  small  to  carry  the  stress  on  it  and  crushed.  A  shear  crack 
cannot  grow  in  this  manner.  It  is  well  known  that  the  maxi- 
mum shear  stress  does  not  occur  at  any  fiber  near  the  extreme 
top  or  bottom  of  a  beam.  Therefore,  when  the  crack  extends 
up  into  the  stem  and  reaches  the  neutral  axis,  the  shear  resist- 
ance of  the  beam  is  practically  exhausted. 

"  The  beams  also  made  it  evident  in  other  ways  that  no  ver- 
tical shear  was  active.  In  some  cases  the  beams  had  received 
a  vertical  crack  in  handling,  the  crack  being  located  about  3  in. 
inside  the  support,  and  extending  clear  through  the  concrete. 
At  first,  it  was  believed  that  these  beams  would  not  give  a  fair 
test,  and  it  was  taken  under  consideration  to  leave  these  beams 
out.  It  proved,  however,  that  the  crack  closed  up  as  soon  as 
the  load  was  put  on,  and  after  the  load  was  increased  to  a  cer- 
tain amount,  the  cracks  were  hardly  visible,  while  the  final 
failure  took  place  some  distance  from  the  injured  section.  If 
there  had  been  any  vertical  shear  acting  on  the  beam,  the  ulti- 
mate load  would  have  reached  a  comparatively  small  value  only, 
and  in  all  probability  the  injured  section  would  have  sheared 
off  at  once. 

"The  tension  in  the  steel  must  be  constant  from  end  to  end 
of  the  beam  between  the  supports.  The  steel  would  have  a 
tendency  to  pull  out  of  the  overhanging  ends  with  a  force  equal 
to  the  total  pull  in  the  steel,  which  is  the  same  near  the  supports 
as  at  the  center  of  the  beam.  The  overhanging  ends  furnish 
the  necessary  anchorage  for  the  bars  on  account  of  the  grip 
of  the  concrete  around  the  bars,  which  increases  with  the  com- 
pression in  the  concrete,  and,  therefore,  also  with  the  load,  the 
horizontal  cross-bars  giving  the  required  horizontal  restraint  of 
the  concrete  to  produce  the  desired  effect.  The  numerical 
value  of  the  length  of  the  anchorage  may  therefore  be  expressed 
in  figures  by  simply  dividing  the  length  of  the  overhang  into 
the  total  pull  on  the  steel,  the  quotient  giving  the  value  of  the 
bond  in  pounds  per  lineal  inch  of  embedment,  regardless  of  the 
amount  of  steel.  This  figure  is  given  in  the  accompanying 
table,  the  length  of  the  anchorage  being  the  length  of  the  over- 


THE  THEORY  OF  BEAMS  AS  ILLUSTRATED  BY  TESTS     219 

hang,  and  disregarding  the  extra  length  of  the  hook  at  the 
ends  of  the  bars.  Beams  5  and  6  are  not  included  in  the  table, 
as  these  beams  had  an  overhang  of  only  10  in.,  leaving  a  hori- 
zontal space  inside  the  support,  and  this,  of  course,  makes  it 
impossible  to  compare  these  two  beams  directly  with  the  rest. 

VALUES  OF  BOND  OBTAINED 

Total  pull  Bond 

Beam  Type  Overhang  in  steel  per  lin.  in. 

1  A  10  in.  24,500  2,450 

2  B  14  in.  27,200  1,940 

3  C  18  in.  39,900  2,230 

4  A  10  in.  23,300  2,330 

7  A       10  in.       27,300       2,730 

8  B       14  in.       37,400       2,670 

9  C       18  in.       41,600       2,310 

"This  table,  it  is  believed,  is  remarkable  when  the  uniform- 
ity of  the  results  is  considered.  The  beams  tested  here  had 
reinforcement  varying  from  ^  of  1  per  cent,  to  1  per  cent., 
spans  varying  from  74  to  80  in.,  and  tension  stresses  in  steel 
varying  from  27,300  to  79,800  Ibs.  per  square  inch.  It  seems  safe 
to  say  that  these  beams  all  failed  by  sliding  of  the  steel. 

"  So  far,  no  attention  has  been  paid  to  beams  5  and  6.  The 
overhang  for  these  beams  was  10  in.  in  each  case,  the  slab  con- 
tinuing for  a  distance  inside  the  supports.  The  bond  stress 
developed  in  the  overhang,  if  figured  as  for  beams  above,  be- 
comes 3,180  and  3,140  Ibs.  per  linear  inch,  or  quite  high  when 
compared  with  the  results  of  the  table  above.  Remembering, 
however,  that  the  straight  portion  of  the  bar  is  continued  inside 
the  supports  for  a  distance  of  4  and  8  in.,  respectively,  the  bond, 
if  distributed  over  the  total  distance  of  14  in.  for  No.  5  and  18 
in.  for  No.  6,  becomes: 

Total  stress    Bond 
Beam  Type  Overhang  in  steel  per  lin.  in. 

5  B    10"  +  4"  =  14"    31,800    2,270 

6  C    10"  +  8"  =  18"    31,400    1,745 

"  If  any  importance  can  be  given  these  two  isolated  results, 
they  would  show  that  the  bond  inside  the  support  is  quite  as 
effective  as  that  outside  the  support,  but  for  a  short  distance 
only,  and  that  its  value  decreases  rapidly  with  the  distance 
inside  the  support." 


220  REINFORCED  CONCRETE  BUILDINGS 

The  lessons  to  be  drawn  from  these  tests  are: 

(1)  That,  with  the  arrangement  used,  the  presence  or  ab- 
sence of  U-bars  does  not  influence  the  strength  of  the  beam. 

(2)  That  "shear,"   properly  understood,   does  not  exist  in 
beams  of  this  kind. 

(3)  That,    with   proper   arrangement   of   the   end   supports 
and  of  the  anchorage,  such  beams  will  not  fail  until  the  com- 
pressive  strength  of  the  concrete,  or  the  tensile  strength  of  the 
steel,  is  exhausted. 

(4)  That  such  beams  are  rational  structures  capable  of  prac- 
tical and  economical  use. 

(5)  That  the  sliding  resistance  of  the  steel  does  not  depend 
upon  the  number  or  size  of  the  individual  rods,  but  only  upon 
the  anchorage  of  the  group  of  rods,  the  length  of  embedment 
being  much  more  important  than  the  diameter  of  either  each 
rod  or  of  the  group  of  rods. 

Effect  of  Joint  between  Slab  and  Stem,  Tests  by  Professor 
Johnson.  In  connection  with  the  introduction  of  the  Ransome 
Unit  System  (p.  162  ff.)  in  Boston,  a  series  of  very  interesting 
tests  were  made  on  T-beams  of  both  the  monolithic  and  unit 
types,  reinforced  with  straight  bars  only,  and  with  both  straight 
and  bent  bars.  All  the  beams  had  U-bars.  In  the  "Unit" 
beams,  the  slab  was  cast  from  four  to  nine  days  later  than  the 
stem.  A  total  of  twenty-eight  beams  were  prepared,  of  which 
eleven  have  so  far  been  tested,  the  balance  being  held  for  a 
longer-time  test.1  The  beams  were  all  reinforced  with  Ran- 
some steel,  that  is,  cold-twisted  squares.  The  U-bars  were 
round  except  in  Type  C,  where  square  twisted  U-bars  had  been 
used. 

Type  A,  Beams  1,  2,  4,  5,  9,  10,  and  12.     See  Figure  159. 

1  The  authors  are  indebted  to  Prof.  L.  J.  Johnson,  M.  Am.  Soc.  C.  E.,  for 
the  following  data,  and  for  permission  to  publish  the  same.  The  beams  were 
designed  by  Prof.  Johnson,  by  Mr.  J.  R.  Worcester,  M.  Am.  Soc.  C.  E.,  Consult- 
ing Engineer,  by  Mr.  J.  R.  Nichols,  Jun.,  Am.  Soc.  C.  E.,  by  the  Concrete 
Engineering  Co.  of  Boston,  and  the  Ransome  Engineering  Co.  of  New  York, 
each  having  designed  one  series  of  beams  or  contributed  to  the  design  by 
suggestions.  Professor  Johnson,  who  made  the  tests  on  the  testing  machine 
in  the  Harvard  University  laboratory,  expects  to  publish  in  due  season  a 
complete  report  of  both  this  series  and  of  the  long-time  tests.  The  authors 
of  this  present  volume,  eye-witnesses  of  these  tests,  are  solely  responsible  for 
conclusions  reached  herein. 


THE  THEORY  OF  BEAMS  AS  ILLUSTRATED  BY  TESTS     221 


In  the  Unit  beams,  the  top  of  the  stem  was  either  left  fairly 
smooth,  as  it  would  be  in  usual  every-day  practice,  or  corru- 


Toggle  for  lifting  beam 


— 8-8 


Rods 


"13-"^ 


Barsy    \  t    1 4^    ^6*  Stirrups  5^ 


Straight' 


FIGURE  159. 


gated  as  shown  in  Figure  160.     The  ends  of  the  stem  rested  in 
previously  prepared   seats    (Figure    161),   and   the   joints  were 


Y 


FIGURE  160. 


sealed  with  grout  to  ensure  a  similar  action  as  obtained  in  actual 
construction,  where  the  Unit  beam  rests  in  a  pocket  in  the  gir- 
der. Nine  days  after  the  casting  of  the  stem,  the  slab 
was  put  on,  while  in  the  monolithic  beam,  the  entire 
amount  of  concrete  was,  of  course,  deposited  in  the 
forms  in  one  operation. 

Beam  No.  5  was  a  Unit  beam,  with  the  top  of  the 
stem  corrugated.  The  age  of  the  stem  was  forty- 
five  days,  that  of  the  slab  thirty-five  days.  At  a 
total  load  of  12,000  Ibs.  the  first  tension  crack  ap- 
peared near  the  middle  of  the  span.  Inclined  cracks 
became  evident  near  the  ends  under  a  load  of  23,000 
Ibs.;  the  ultimate  load  was  41,000  Ibs.,  when  failure 
occurred,  through  compression  of  the  slab  between 
the  loads,  and  slipping  of  the  straight  tension  bars 
(see  beam  No.  12  below). 

Beam  No.  4  was  a  Unit  beam,  the  top  of  the 
stem  being  fairly  smooth;  that  is,  no  attempt  had 
been  made  toward  getting  a  particularly  .rough  sur- 
face. The  age  of  the  stem  was  forty-five  days,  of  the  slab 


t 

t 

i 

1 

I 

i 

1 

FIGURE  161. 


222 


REINFORCED  CONCRETE  BUILDINGS 


thirty-six  days;  the  first  crack  was  observed  under  a  load  of 
20,000  Ibs.;  ultimate  failure  took  place  under  48,000  Ibs.  in 
precisely  the  same  manner  as  in  No.  5. 

Beam  No.  1  was  exactly  similar,  except  that  slab  and  stem 
were  both  one  day  older  than  in  No.  4;  the  ultimate  load  was 
49,400  Ibs.,  and  the  beam  failed  in  the  same  manner  as  the  fore- 
going. 

Beam  No.  2  was  of  the  same  age  and  detail  as  No.  1 ;  the  first 
crack  was  observed  at  10,000  Ibs.  loading;  the  ultimate  failure 
occurred  in  the  same  manner  as  above  under  54,300  Ibs.  total. 
The  higher  load  on  this  beam  is  perhaps  due  in  some  measure 
to  the  fact  that  the  rocker-supports  for  the  beam  came  to  a 
bearing,  making  possible  some  horizontal  thrust  on  the  beam. 

Beam  No.  12  was  monolithic,  forty-one  days  old;  of  same 
design  as  the  foregoing  Unit  beams,  except  that  the  fillet  between 
stem  and  slab  was  slightly  reduced  (see  Figure  162).  The  first 


FIGURE  162. 

crack  occurred  at  4,000  Ibs.,  the  ultimate  load  was  45,800  Ibs., 
and  failure  occurred  through  a  slip  of  the  straight  reinforce- 
ment, causing  the  sudden  collapse  of  the  left  end. 

Beam  No.  9  was  of  the  same  general  design,  cast  in  one 
piece,  and  forty-one  days  old.  The  first  crack  occurred  at 
3,000  Ibs.;  the  beam  failed  suddenly  at  50,000  Ibs.  by  slipping 
of  the  rods  at  the  right  end. 

Beam  No.  10  was  also  a  monolith  forty-one  days  old,  show- 
ing a  tension  crack  at  6,000  Ibs.,  with  ultimate  failure  at 
52,400  Ibs.  from  a  combination  of  initial  sliding  of  the  tension 
rods  with  compression  at  the  center. 

It  will  be  seen  from  these  data  that  the  Unit  beams  stood  up 
as  well  under  the  load  as  the  monolithic  beam,  so  that  the  joint 
between  slab  and  stem  was  perfectly  adequate,  whether  cor- 
rugated or  plain.  The  general  behavior  of  all  these  beams  up 
to  the  point  of  failure  was  so  much  the  same  that  no  one,  from 


THE  THEORY  OF  BEAMS  AS  ILLUSTRATED  BY  TESTS     223 


observation  of  the  beams  in  the  machine,  could  have  pointed  out 
which  beams  were  unit  and  which  monolithic.  In  fact,  they  all 
failed  in  the  customary  manner,  exhibiting  the  usual  inclined 
and  vertical  cracks,  and  no  sliding  was  noticeable  between  slab 
and  stem,  although  carefully  looked  for. 

Type  B,  Beams  25  and  27.     See  Figure  163. 


2-3/-  Bars'T  t   I  Ho  •Stirrups 

2-K"«  Straight 


Ho  •  Stirrups  5K£ 
FIGURE  163. 

The  beams  of  Type  B  were  built  exactly  as  the  beams  of 
Type  A,  except  that  the  tension  rods  had  been  reversed,  being 
2|"  bars  bent  and  2^"  bars  straight.  This  reinforcement 
would,  under  the  theories-  advanced  in  this  book,  be  more  effi- 
cient, and  the  U-bars  were  therefore  reduced  from  TV'  round 
stock  in  Type  A  to  i3s  "  round  stock  in  Type  B,  thus  having  about 
one-third  of  the  area  of  the  former. 

Beam  No.  25  of  Unit  construction  had  a  stem  twenty-nine 
days  old  and  a  slab  twenty-five  days  old;  the  first  crack  was 
observed  under  a  load  of  9,000  Ibs.,  and  ultimate  failure  took 
place  under  simultaneous  compression  of  the  slab  and  of  the 
side  of  the  stem,  at  the  point  where  the  tension  rod  was  bent, 
under  a  load  of  42,500  Ibs.  (See  Figure  164.) 


2  -«•  Straight 


FIGURE  164. 


Beam  27 


Beam  25 


Beam  No.  27  was  monolithic,  of  same  design,  and  twenty- 
seven  days  old.  The  first  crack  was  seen  at  10,000  Ibs.  ;  while 
the  ultimate  load  was  47,500  Ibs.  Also  in  this  case  was  com- 
pression in  both  slab  and  stem  evident  as  shown  in  Figure  164. 


224 


REINFORCED  CONCRETE  BUILDINGS 


It  is  interesting  that  these  two  beams  carried  practically 
as  much  load  as  the  older  beams  of  Type  A,  in  spite  of  the  great 
reduction  in  the  weight  of  the  U-bars.  The  explanation  is  to 
be  found  in  the  theory  set  forth  in  Chapter  VII  of  this  book, 
where  the  relation  between  the  bent  bars  and  the  U-bars  has 
been  considered  at  length;  —  in  fact,  the  design  of  beams  25 
and  27  was  made  to  prove,  or  disprove,  these  theories  as  far  as 
possible. 

Type  C,  Beams  13  and  20.     See  Figure  165. 


i       /i  »     • 

1 ,,0-10— ,,-J 

'oggle  f orjif ting  beam  k-8   >r<  8— >i 

9_  S/' 


1 
-f 

*^ 

/8-X  •  Rods 

_^. 

rT"~T~7~r~7~1 

i     !     i     !     ,     !     !     ! 

i 

Vf7 

J.-L_L_L_L 

_o_L  —  1  —  X  —  i  —  J  —  J  —  J  —  J  

._  1 

-L/ 

3 

CO 

i 

V////\ 

V  2-1"  « 

w/ 

y///k 

H 

"•  Stirrups  *'Y 

*Y///A 

FIGURE  165. 

In  designing  these  beams,  Professor  Johnson  had  endeavored 
to  secure  a  high  strength  in  compression  and  tension.  The 
special  feature  was  the  absence  of  bent  bars  so  that  the  stresses  in 
the  stem  and  in  the  joint  between  slab  and  stem  were  especially 
severe  under  the  common  theory  of  shear.  The  beams  rested  in 
concrete  supports  shown  in  Figure  166. 

Beam  No.  20  was  of  the  Unit  type  with  corrugated  top 
(Figure  167).  The  stem  was  forty-two  days  old,  the  slab  thirty- 


4 

1  i  "^  i  1           1 

%" 

_/    \  /     \  /  '  \  /  j 

t 

i  Wj 

[      !      ! 

FIGURE  166. 


FIGURE  167. 


two  days  old.  Tension  cracks  developed  in  the  usual  manner, 
beginning  under  a  load  of  10,000  Ibs. ;  at  22,000  Ibs.  small  diag- 
onal cracks  appeared.  At  50,000  Ibs.  it  was  noticed  that  the 
visible  end  of  the  curved  tension  rods  began  to  slide,  and  the 


THE  THEORY  OF  BEAMS  AS  ILLUSTRATED  BY  TESTS     225 

ultimate  failure   occurred   under  a  load  of  55,600  Ibs.,   when 
the  compression  area  was   crushed. 

Beam  No.  13  was  again  of  the  Unit  type,  with  smooth  top  of 
stem,  which  was  forty-three  days  old;  the  slab  was  thirty-four 
days  old.  The  first  tension  craok  occurred  at  9,000  Ibs.,  and 
the  cracks  then  developed  in  the  usual  manner.  Failure  took 
place  at  50,000  Ibs.,  when  the  adhesion  between  concrete  and 
steel  was  broken;  the  rods  began  to  pull  through,  and  the  slab 
was  crushed  at  the  center. 

The  analysis  of  the  stresses  follows: 
Types  A  and  B 

By  reference  to  formula  (18),  page  38,  we  have 

2  =  1;  T  =  4";  H  =  9";  D  =  13";  V  =  ~  X  1.62  =  2 

hence 

S  82.0 


and 

^  =  7^M=^  =  -436;1-?  =  -8^ 


Now,  the  bending  moment  is  f  •  L.42  =  21  L,  and  the  arm  of 
internal  stresses  approximately 

.855  X  13  =  11.1  inches,  hence  the  pull  in  the  steel 
s  =  jj-L  =  1.89Llbs. 

The  beams  had  1.62  square  inches  of  tension  steel,  hence  the 
unit  tension  on  steel: 

S       L89    T        1  17    T 
=  'L  =    1'17'L 


and  the  unit  compression  on  the  concrete 


Type  C 

|  =  i  ;  T  =  5";  H  =  6";  D  =  11";  V  =  15  -  ^  =  2.5; 


226  REINFORCED  CONCRETE  BUILDINGS 

hence 

~  =  15.9 
and 


x  = 


1  + 


15.9 
15 


-  .485;  1  -  l-x  =  .838 


Again,  the  bending  moment  is  \  -L-30  =  15  L,  and  the  arm  of 
internal  stresses  approximately 

11  X  .838  =  9.2",  hence  the  pull  in  the  steel  is 


The  beams  had  2.0  square  inches  of  steel,  hence  the  unit  tension 
on  steel 


and  the  unit  compression  on  the  concrete 


It  is  evident  that  these  calculations  do  not  give  the  true 
stresses  existing  at  rupture,  because  r  is  not  equal  to  15  at  that 
time,  and  the  assumption  of  plane  sections  probably  does  not 
hold  good.  For  the  sake  of  comparison,  however,  they  may  be 
useful.  The  results  are  indicated  in  the  table.  The  testing 


^ 

Corresponding 

« 

6 

Age  (days) 

calculated  stresses 

"SI 

fc 

Ultimate 

165  sq.  in. 

a" 

S 

How  made 

Load 

>•£ 

2 

Ibs. 

1 

pq 

Stem 

Slab 

C 

s 

A 

1 

Unit,  Smooth  Top 

46 

37 

49,400 

3060 

57,800 

A 

2 

Unit,  Smooth  Top 

46 

37 

54,300 

3370 

63,500 

A 

4 

Unit,  Smooth  Top 

45 

36 

48,000 

2980 

56,200 

A 

5 

Unit,  Corrugated  Top 

44 

35 

41,000 

2540 

48,000 

A 

9 

Monolith 

41 

41 

50,000 

3100 

58,500 

A 

10 

Monolith 

41 

41 

52,400 

3240 

61,200 

A 

12 

Monolith 

41 

41 

45,800 

2840 

53,600 

B 
B 

25 
27 

Unit,  Smooth  Top 
Monolith 

29 
27 

25 

27 

42,500 
47,500 

2640 
2950 

49,700 
55,600 

C 

13 

Unit,  Smooth  Top 

43 

34 

50,000 

2560 

40,700 

C 

20 

Unit,  Corrugated  Top 

42 

32 

55,600 

2840 

45,300 

Harvard  Test  Beams.    Summary  of  Results  obtained 


THE  THEORY  OF  BEAMS  AS  ILLUSTRATED  BY  TESTS     227 


machine  was  equipped  with  means  for  registering  the  deflec- 
tions automatically;  the  diagrams  are  shown  in  Figures   168, 


FIGURE  168. 

169,  and  170.     Generally  speaking,  there  is  little  difference  in 
the  deflection  of  the  Unit  and  monolithic  beams. 

A  number  of  interesting  observations  were  made  during 
these  tests.  First,  the  feasibility  of  the  Unit  beam  was  estab- 
lished beyond  doubt,  contrary  to  what  many  engineers  would 


Deflection 

^L 

40000 



^25 

^<^ 

V 

30000 

> 

\ 

20000 

\ 

\ 

1.0* 

0.9" 

0.8  " 

0.7" 

0.6" 

0.5" 

0.4" 

0.3" 

0.2" 

0.1"   \ 

FIGURE  169. 

probably  have  expected.  In  fact,  many  building  regulations 
throughout  the  country  specify  positively  that  the  beam  and  its 
superimposed  slab  must  be  concreted  in  one  continuous  opera- 


228 


REINFORCED  CONCRETE  BUILDINGS 


tion.  Where  improperly  designed,  or  otherwise  inadequate, 
U-bars  are  used,  this  rule  is  undoubtedly  highly  beneficial,  but 
where  proper  U-bars  are  used,  the  rule  is  wholly  unnecessary. 
The  progress  report  of  the  special  committee  of  the  American 
Society  of  Civil  Engineers  recommends  that  the  slab  be  con- 
sidered effective  in  compression  when  " proper  bond"  is  pro- 
vided between  slab  and  stem;  it  will  be  appreciated  that  this 
is  a  much  more  consistent  requirement,  although  somewhat 
indefinite.  The  beams  tested  so  far  have  shown  that  the  bond 
provided  was  adequate,  whether  the  more  elaborate  method  of 


Deflection    0.9"        0.8" 


50000 


30000 


0.6' 


0.5' 


13 


FIGURE  170. 

corrugating  the  top  of  the  stem  was  used,  or  whether  the  top  of 
the  stem  was  simply  left  as  it  was  upon  completion.  It  would 
be  very  interesting  to  learn  what  would  happen  when  the  bond 
was  "  inadequate,"  and  just  where  the  limit  may  be  found,  and 
in  this  particular  the  present  tests  furnish  no  information,  as 
the  bond  remained  intact  in  all  cases.  See  Figures  171  and  172, 
showing  the  Unit  Beam  No.  25. 

In  the  second  place,  these  tests  confirm  in  a  remarkable 
degree  the  theories  set  forth  by  the  author  in  Chapter  VII  in 
regard  to  the  action  of  U-bars. 

Compression  failures  of  the  stem  were  observed  in  beams  25 
and  27;  these  are  shown  in  Figure  164  and  in  Figures  171-174. 
It  was  observed  that  the  compression  failure  of  the  stem  was 
on  the  same  side  as  the  corresponding  bent  bar,  the  two  bent 
bars  being  each  near  the  opposite  face  of  the  beam ;  in  beam  27 


THE  THEORY  OF  BEAMS  AS  ILLUSTRATED  BY  TESTS     229 


FIGURE  171.    HARVARD  BEAM  No.  25. 

The  black  lines  are  ink  marks  indicating  the  principal  cracks 

Photo  by  Mr.  J.  R.  Nichols,  Jr.,  Am.  Soc.  C.  E. 


FIGURE  172.    A  CLOSER  VIEW  OF  BEAM  No.  25,  SHOWING  CRUSHING 
OF  THE  CONCRETE  AT  THE  ROD. 

This  beam  was  a  Unit  beam,  —  it  will  be  noticed  that  there  was  no  indication 
of  slipping  between  stem  and  slab. 

Photo  by  Mr.  J.  R.  Nichols,  Jr.,  Am.  Soc.  C.  E. 


230 


REINFORCED  CONCRETE  BUILDINGS 


FIGURE  173.     BEAM  No.  27,  SHOWING  PRINCIPAL  CRACKS  AT  LEFT  END, 
AND  THE  CRUSHING  OF  THE  CONCRETE  AT  THE  ROD. 

Photo  by  Mr.  J.  R.  Nichols,  Jr.,  Am.  Soc.  C.  E. 


FIGURE  174.    CLOSER  VIEW  OF  BEAM  No.  27. 

Photo  by  Mr.  J.  R.  Nichols,  Jr.,  Am.  Soc.  C.  E. 


THE  THEORY  OF  BEAMS  AS  ILLUSTRATED  BY  TESTS     231 

crushing  took  place  at  both  bent  bars,  one  spot  on  each  side, 
but  in  different  locations,  corresponding  to  the  position  of  the 
curves  in  the  bars.  It  is  self-evident  that  this  upward  pres- 
sure of  the  rod  must  be  resisted  by  an  equal  downward  pres- 
sure (from  the  load)  thus  dissolving  the  beam  into  a  number  of 
well-defined  compressive  zones  in  a  manner  very  different  from 
what  takes  place  in  a  " solid"  homogeneous  beam.  The  same 
observation  was  made  by  Prof.  Morsch  in  regard  to  his  test 
beam  No.  VI. 

Third,  a  deep,  gaping  crack  was  observed  in  the  top  of  beam 
No.  20  (Figure  175),  near  the  support.     The  explanation  of  this 


FIGURE  175. 

crack  may  be  found  in  the  distribution  of  internal  stresses 
indicated  in  the  drawing,  the  horizontal  arrow  at  the  steel  indi- 
cating the  pulling  of  the  steel,  the  inclined  arrow  indicating  the 
sum  of  the  compressive  forces  in  the  concrete.  It  will  be  noted 
that  if  these  two  do  not  intersect  on  the  vertical  line  of  the 
reaction,  a  "  re  verse"  bending  moment  is  created  at  the  end 
which  would  cause  just  such  a  crack.  Here  again  we  have  a 
fact  showing  that  a  reinforced  concrete  beam  cannot  be  consid- 
ered as  a  " solid"  beam,  in  which  such  stresses'  are  impossible. 
Considering  the  beam  as  a  truss,  we  see  at  once  that  the  crack 
comes  outside  the  "end  panel,"  and  so  would  have  no  influence 
on  the  load-carrying  capacity. 

In  addition,  it  must  be  admitted  that  "shear,"  so  called, 
would  have  caused  the  instantaneous  collapse  of  a  beam  with 
such  a  crack.  As  an  actual  matter  of  fact,  this  beam,  with  the 
gaping  crack  in  the  top,  carried  a  total  load  of  55,600  Ibs.,  or 
more  than  any  other  beam  of  the  entire  series.  The  stem  was 
perforated  with  inclined  and  vertical  cracks  so  that  the  only 
portions  of  the  beam  which  could  actually  carry  some  shear 
were  the  main  tension  rods.  This  proposition  has  been  consid- 
ered above  and  cannot  be  maintained.  The  truth  is  that  there 


232 


REINFORCED  CONCRETE  BUILDINGS 


was  no  active  shear  in  this  beam,  the  system  consisting  approx- 
imately of  members  as  shown  in  Figure  176.1 

Fourth,  it  was  established  that  the  quarter  turn  given  the 
straight  tension  rods  at  the  ends  was  not  sufficient  to  develop 
the  desired  amount  of  sliding  resistance.  Thus,  beams  9  and  12 
failed  suddenly  by  the  entire  separation  of  the  lower  rods  from 
the  concrete,  while  the  beams  of  Type  C  (13  and  20)  showed  a 
sliding  of  from  i"  to  f "  (in  these  beams,  the  ends  of  the  rods 

I          I 


FIGURE  176. 

could  easily  be  observed  by  breaking  away  a  thin  shell  of  con- 
crete). The  behavior  of  the  balance  of  the  beams,  and  especially 
inspection  of  the  deflection  diagrams,  makes  it,  however,  appar- 
ent that  only  a  very  small  additional  margin  of  sliding  resistance 
was  required  in  order  to  prevent  the  sudden  collapse.  Without 
doubt,  the  large  turn  of  the  upper  bar  might  profitably  have 
terminated  at  its  lowest  point,  as  the  last  fourth  of  the  circle 
materially  weakened  the  concrete  along  the  lines  of  cleavage 

1  The  authors  are  aware  of  the  fact  that  other  observations  were  made 
during  the  testing  of  the  Harvard  series  which  strongly  support  the  theory 
advanced  in  Chapter  VII  of  this  volume.  We  are,  however,  requested  to 
withhold  this  matter  from  publication  at  the  present  time,  and  we  must 
refer  to  the  later  report  to  be  published  by  Prof.  Johnson. 


INDEX 


Accidents,  191 

Acid,    carbonic,   for  hardening   con- 
crete, 12 

hydrochloric,    for    joining    con- 
crete, 10 

joint,  10 

joint,  specifications  for,  198 
Adhesion,  54 

Allowable  stresses,  52,  128 
Alum,  190 

Arches,  allowable  stresses  in,  131 
Assumptions,  homogeneity,  51 

in  general,  52 

tensile    resistance    of    concrete 
disregarded,  104 

Basic  inventions,  18 

Beam  formulas,  70,  71,  75,  88 

Belt  course,  Ransome  patent,  13 

reinforcement  of,  153 
Bending,  66 

combined  with  compression,  114 
Board  marks,  176 
Brushing,  179 

Cement,  137 

Chimneys,  approximate  formula  for, 
115 

Chloride  of  calcium,  2, 190 
of  sodium,  2,  190 

Clay,  effect  on  concrete,  12 

Coil  joint,  concrete  to  concrete,  9 
for  joining  rods,  15 

Columns,  allowable  stresses,  130 
hooped,  58 
least  diameter,  62 
repairs  of  defective,  189 
strengthening  existing,  64 


Concrete,  dry  or  wet  mixture,  16 
mixing  and  placing,  149 
shrinkage  and  swelling,  126 
specifications,   196 

Conflagrations,  185 

Continuity  of  beams,  109 

Core  boxes,  permanent,  15 

Corners  chamfered,   176 

Cracks,  in  cement  finish,  181 
in  slabs  and  beams,  127 
in  tile-concrete  floors,  182 
repairs  of,  188 
structural  significance,  92 

Delayed  placing  of  concrete,  7 
Design,  general  remarks,  153 

Earthquake,  San  Francisco,  6 
Embedment,  required  length  of,  55 
Expansion  joint,  2,  128 

Facing  of  concrete,  177 

see  also  Veneer 
Factor  of  safety,  129,  132 
Falsework,  general  design,  156 

instructions,  196 

standardization,  15 
Finish,  cement,  general,  181 

instructions,  198 
Fireproofing,  183 

effect  of  salt,  16 
Floor  coverings,  cement  finish,  181 

wood,  154 
Footings,  formulas,  116 

foundations,  171 
Forms.     See  Falsework 
Frost  protection,  151 

effect  of  salt,  10 


233 


234 


INDEX 


Hooked  ends  of  reinforcement,  55 
Hooped  columns,  58 

Inertia,  moment  of,  112 
Injurious  agencies,  17 
Illuminating  panels,  8 

Joining  new  concrete  to  old,  9,  198 

Laitance,  149,  181,  189 

Lateral  expansion,  57 

Lime,  slacked  lime  in  concrete,  12 

Mixing  and  placing  of  concrete,  149 
Molds.     See  Falsework 
Monolithic  construction,  156 

Notations  used  in  bending  theory,  66 
Overmixing  of  concrete,  7,  8 

Patentees,  Alsip,  31 
Aspdin,  19 
Basset,  33 
Bissell,  35 
Brannon,  23 
Bruner,  39 
Bunnet,  21 
Cheney,  32 
Coddington,  24 
Coignet,  22,  30,  31 
Considere,  46 
Cornell,  28 
Cottancin,  40 
Cubbins,  36 
De  Man,  41 
Dennet,  21 
Edwards,  26 
Emerson,  33 
Emmens,  24 
Fowler,  29 
Gedge,  22 
Gilbert,  30 
Golding,  36 
Gustavino,  38 
Hallberg,  44 
Henderson,  31 
Hennebique,  42 


Patentees,  Hyatt,  23,  24,  25,  26,  33, 
34,  35 

Jackson,  32,  36,  37,  38 

Johnson,  22,  29,  40 

Kahn,  45 

Knight,  28 

Lambot,  27 

Lish,  24 

Lythgoe,  22 

McCarthy,  40 

Matrai,  43 

Matthews,  33 

Melan,  40 

Middleton,  28 

Monier,  22,  27,  38 

Parker,  19 

Parkes,  22 

Parmley,  45 

Rabitz,  39 

Ranger,  21 

Ransome,  E.   L.,   2,  3,  5,  8,  9, 
10,  13,  15,  16 

Ransome,  Fk.,  22 

Shaler,  43 

Sisson,  32 

Smith,  31 

Stempel,  39 

Stephens,  28 

Stevenson,  28 

Summer,  28 

Tall,  23 

Thacher,  43,  44 

Thornton,  22 

Turner,  24 

Visintini,  45 

von  Emperger,  40 

Waite,  41 

Wayss,  44 

Weber,  46 

Wetmore,  32 

Wilkinson,  21 

Williams,  30 

Wilson,  39 

Wood,  30 
Wyckoff,  28 
Piling,  171 

cost  of,  174 
Plastering,  176 


INDEX 


235 


Plates,  concrete,  118,  120 
steel  base,  118,  171 

Reinforced  concrete,  defined,  51 

elements  of  invention,  18 
Reinforcement,  details  of,  105 

double,  114 

circular,  118 

kinds  of,  145,  147 

requirements,  147 

placing,  159 

Repairs  to  buildings,  188 
Rolling  of  floors,  8 

instructions,  196 
Rubbing  of  surfaces,  179 

Salt,  effect  on  concrete,  10,  184 

instructions  for  using,  196 
Sand,  143 
Sidewalk  lights,  8 
Silicate  of  lime,  2 

of  potash,  190 

of  soda,  2,  190 
Slab  formulas,  79 
Slag,  aggregate,  145 

for  making  joints,  9 
Sliding,  of   concrete  upon   concrete, 
103 

of  reinforcement,  101 

see  also  Adhesion,  54 

effect  of  U-bars,  57 
Specifications,  superintendents',  195 
Stand-pipes,  132 
Stone,  144 
Steel,  145 

specifications,  198 

see  also  Reinforcement 


Stresses,  longitudinal,  66 

transverse,  93 

tensile  (in  concrete),  104 

initial,  126 

temperature,  126 

allowable,  128 
Stirrups.  See  U-bars 

Tables,  T-beams  of  minimum  depth, 
72,  73 

T-beams  of  increased  depth,  78, 
80 

tile  concrete  floors,  78 

discussion  of,  81 
Tanks,  132 

Tensile  stresses  disregarded,  104 
Tile-concrete  construction,  76 

tables,  78 

description  of,  148 
Tooling,  178 
Twisted  bars,  invention  of,  3 

effect  of  twisting,  147 

U-bars,  97 

spacing  of,  102 

Unit    construction,    Ransome's,    16, 
162 

types  of,  161,  162 

Veneered  buildings,  187 

Water,  consistency  of  concrete,  149 
consistency  of  finish,  181 
dry  versus  wet  concrete,  16 
hot  water  used,  21 
importance  of  sprinkling,  127 


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